;=",~c BUGLe, ",._- ... ' '-'--"t S1.~TrVJ(~";';>;' L-E :~-' ~'/\ -'-.\ -, i~'-:;AS

,-," ~< ,r-,--" , PRO V I Nte+A"OEMO GAMBIQU E'

SERVIC;OS DE GEOLOGIA E MINAS

ESTUDOS, NOTAS E TRABAlHOS DOS

SERVICOS DE GEOlOGIA E MINAS

B 0 LET I M N. 0 2,9

1 9 6 2 EMPRESA MODERNA, LOA. LOUREN,O MARQUES

ERRATA

~\t' !' .-., t<'"'STmIT"'UT"'E--'" • Of

GEOLOGICAL / SCIENC£?-/

FAg. 7, linhas 11' e 28, onde se 11) "Imbamba Su1" deve ler­-se "Lubemba Sul".

fNDICE

(I) - Amonites do CreMcico inferior do rio Maputo (Catuane HMo9ambique). por

C. Hentiques da Silva

(2) - Weathering and climatic environment 01 a basic lava of the Lebombo near

Pag_

5

Moamba, Province of MOfambique, by H. H. Weinert and K. A. Clauss ... 3S

(3) - Nota sabre 0 cstado de uma anlibala de M09ambique pelos metodos radiocrisH

talogr6.ficos, por Maria Irene Fernancles Vaz 47

(2 )

Weathering and Climatic environment of a basic lava of the lebombo, near Moamba,

Province of Mo~ambique

BY

H. H. WE INERT, PH. D.

Senior Research Of/icer, NutiaM! 1,!stifu!c for Road Research, South

African Council for Scientific and Indusidol Research

AND

K. A. CLAUSS

Research Officer, National Institute for Road Research, South

African. Council for Scientific and Industrial Research

WEATHERING AND CLIMATIC ENVIRONMENT OF A BASIC LAVA OF THE LEBOMBO NEAR MOAMBA, PROVINCE OF MO~AMBIQUE

by

H. H. Weinert and K. A. Clauss

SUMMARY

A basic and an intermediate lava of the Lebombo. from a site 8 km west of Moamba in the Province of Mo~ambique. are described petrogra­phically. showing in particular that these lavas do contain zeolite.

The terms "disintegration" and "decomposition" and "fresh", "weathe .. red". "badly weathered" and "residual soil" are defined. The samples des­cribed are 'classified in the last three of these categories. Montmorillonite is the principal secondary mineral and it is found in all samples investigated. No kaolinite occurs.

The climatic conditions which influence weathering ·have been inves­tigated and a formula for their quantitative evaluation is presented. It is shown that the mode of weathering near Moamba agrees with the existing climatic conditions.

INTRODUCTION

The main road to Louren~o Marques from Ressano Garcia. on the South African border. crosses an apparently unnamed tributary of the Inko­mati River approximately 8 km west of Moamba. The stream flows very close to the contact between rhyolites and underlying basic lavas of the Lebombo. The rocks exposed in the stream bed are basic to intermediate and no acid rock occurs on the site. Reddish-brown residual soil over lies the lava in protected places. Fig. I is a sketch of the site.

De Assun~ao. Coelho and da Rocha (I) have very recently. published a detailed description of the Lebombo lavas. and a number of other authors have also done so since 1874. This paper. therefore. deals only with the weathering of such basic lavas in their climatic environment at the above­mentioned site.

36 BOLETIM DOS SERVICOS DE GEOLOGIA E MINAS

Definition of Terms

Since terms are often used indiscriminately. several defini~ions were established during a study of the engineering properties of basic igneous rocks in South Africa, particularly in regard to Karroo dolerites (2), which are of special practical importance to the road engineer (3). These definitions are as follows:

Weathering. This is the process of alteration of rocks occurring under the direct influence of the hydrosphere or atmosphere.

Disintegration. This is the result of physical action on a rock, leading to its breakdown without actual chemical alterations of the minerals.

Decomposition. This is caused by chemical action, leading to alte­ration of the rock's minerals. The most conspicuous result of this process is the formation of new "secondary" rninerals.

Secondary minerals. In this paper these comprise only those minerals which originate when a rock weathers under the influence of the hydros­phere or atmosphere; they do not include those minerals which are deriva­tives of the original minerals in the rock due to internal processes.

Most secondary minerals, in this sense, are clay minerals, and the determination of their quantity has been used to determine the engineering soundness of rocks (4), (5). The quantity of secondary minerals is deter­mined under the microscope by counting on a grid pattern, and the results are an indication of the degree of decomposition of rocks other than sedimentary.

Following on the above, a fresh rock is a rock which is completely unaffected by the hydrosphere or atmosphere, consisting exclusively of those minerals which originated during its formation either as an igneous. a sedimentary or a metamorphic rock. This definition of "fresh" is very wide, because many of the fresh sedimentary rocks consist of minerals which would be typical of one or other degree of weathering if they were to occur in igneous or metamorphic rocks. The term "fresh", therefore. must be defined separately for each of the great groups in rock classification.

It is realised that a fully "fresh" rock on, or very near the surface, will only be found in exceptional cases. A certain degree of alteration of some of the rock's original minerals will always have occurred and the definition of "fresh" will, therefore, be based on the quantity of secondary minerals and the consistence of the rock, the former being more important than the latter.

The following characteristics refer to igneous rocks: The microscopic appearance of a fresh rock is characterized by clear

colours, and well developed mineral faces. The rock is hard and rings

~STUOO$. NOT AS E TRABALHOS DOS SERVICOS DE CEOLOCIA E MINAS 37

when struck with a hammer. The microscopic investigation must show not more than 15 % of secondary minerals if the rock is to be classified as fresh. The engineering performance of such rock under all environmental conditions will be sound.

The macroscopic appearance of a weathered rock differs from the fresh one by the dull colour and the washed or "eaten" mineral faces. The rock is still hard. but does not ring when struck with a hammer. Such rock contains 15 % to 35 % secondary minerals. and its soundness for engineer-. ing purposes will depend on a certain relation between the quantity of secondary. miner~ls and environmental conditions.

Badly weathered rock appears in the field as a yellowish. brownish or grey mass which is extremely brittle or even soft. Only quartz and muscovite may still show clear mineral faces. yet the structure of the original rock is still visible. Microscopic inspection reveals more than 35 % secon­dary minerals. The mafic minerals and plagioclase do not occur in the "badly weathered" stage and montmorillonite and kaolinite occur in their place. Such rocks will perform unsoundly under many different environ­mental conditions.

Residual soil is the last stage of decomposition where either montmoril­lonite or kaolinite and. if conditions are favourable •. allitic compounds predominate. The percentage of secondary minerals is more than 50 % in most cases. no definite limit between the percentage for badly weathered rocks and residual soil can, however. be drawn at this stage.

The Weathered Basic Lavas from Moamba

The sampling site is about 8 km west of Moamba where the main road crosses a tributary of the Inkomati River (see Fig. 1). The tributary has cut its bed into the country rock which is basic to intermediate lava of the Lebombo. under patches of residual soil intermixed with blown and washed-in sand.

Three samples were collected as tabulated below:

TABLE I: Description of samples collected.

Sample Description of sample Place of collection No.

172 Residual soil Southern waH of road cutting east of bridge.

173 Badly weathered lava 30 m south of bridge just above high water

mark.

174 Weathered lava 30 m south of bridge below high water mark.

88 BOLETlM DOS SERVICOS DE GEOLOGIA E MINAS

The sample numbers refer to the collection of the National Institut~ for Road Research. Pretoria.

The samples. were analysed by the National Chemical Research Labo­ratory. Pret~ria. and the information is given in Table 2. P, 0, and CO, were not determined.

TABLE 2 : Chemical analyses and secondary minerals 0/ the sample Nos. 174. 173 and 172.

Basic lava IntcrmeBiote Residual

Sample No. lava soil 17' 173 172

Chemical analysis:

SiOz . ". ... ... ... ... ... .. . .. , 45.97 % 54.64 % 49.42 % AlzOs . ... ... ... ... ... ... .. . .. , 12.85 % 13.70 % 14.69 % Fe2OS

' ... ... ... ... . .. ... .. . .. , 8.86 % 13.19 % 9.08 % FeD. ... ... ... ... ... ... .. . ..' 7.04 % 0.85 % 1 .. 10% MgO ". ... ... ... ... ... .. . .. , 6.24 % 2.72 % 3.52 % CaD ... ... ... ... .. . ... '" ... 9.49 % 4.30 % 8.23 % NazO ". ". ... ... ... ... .. . .. , 1.46 % 2.29 % 1.81 % Kp .. ". ... . .. ... ... .. . .. . .. , 0.66 % 0.98 % 1.40 % TiOz ". ... ... ... '" ... . .. ..' 2.32 % 1.84 % 2.03 % Hp ... ... ... ... ... .. . .. . .. , 4.82 % 6.09 % 8.42 %

Total ... ... ... ... 99.71 % 100.60 % 99.70 %

Lacroix paramewic expressions: ... ... .. , III 04/1, 4 (5) .4 11. 3 (4).3 (4).4 -Secondary minerals: .. ... ... ... ... .. . 26.2 % 54.0 % 62.0%

Classified stage of weathering: ... ... ... Weathered Badly Residual weathered soil

Sample No. 174: Basic lava

This is an amygdaloidal rock of dull pinkish-brown colour. The matrix is dense to microscrystalline with some volcanic glass. Plagioclase needles up to 3 mm long appear in the ground mass.

Creen spots appear on the margins of the amygdales and are irregu­larly spread over the rock. X-ray investigations showed that the green margins of the cavities are most probably caused by the green' variety of pyrophyllite. while the green spots in the matrix may be due to saus­suritisation.

The rock is rich in amygdales of varying sizes and diameters of up to nearly I cm were measured. The volume of the amygdales is between 5 and 10 % of the total volume of the rock. They are filled with white to colourless glassy minerals of partly radial acicular and partly fibrous texture.

X~ray and microscopic investigations showed that the rock consists of plagioclase. bytownite in particular. pyroxene in the form of pigeonite. and - especially in the amygdales - thomsonite. pyrophyllite. most pro-

•

ESTUOOS, NOTAS E TRABALHOS DOS SERVICOS DE GEOLOGIA E MINAS 39

bably apophyllite and zeolites of the scolecite and heulandite groups. Weathering products in the rock are montmorillonite in remarkable quan­tities. The rock contains no quartz.

According to .W. von Engelhard (6), 1954, the area under the peak in a Xwray diagram.can be used as an approximate expression for ~he relative quantities of the minerals in a rock. The areas obtained in sample No. 174 were:

Quartz Plagioclase Pyroxene .. Zeolite Montmorillonite

area

o mm' 575 mm' 160 mm' 9Omm2

345 mm'

The intensity of the remaining minerals could not be determined with certainty because of the mutual overlapping of their peaks with those of the zeolites.

The chemical analysis shows a slight oversaturation (see Table 2) and the Lacroix parametric expression for the rock is Ill. 4".4 (5). 4.

The high content of montmorillonite and the relatively high percentage of crystal water indicate the degree of weathering. The quantity of secon­dary minerals was found to be 26.2 % and it can be assumed that they are almost exclusively montmorillonite. The percentage of secondary minerals classifies the rock as "weathered".

Sample No. 173: Intermediate lava:

This is an amygdaloidal rock from a later lava flow, the colour of which is much more brown than the one of sample No. 174. The matrix is similar to that of No. 174, but numerous amygdales are not filled. Some of them were never filled, thus being gas hollows, while others were most likely filled at one time but the original contents have now disappeared. The number of cavities is smaller than in sample No. 174.

The mineral composition of the rock is again plagioclase, pyroxene, zeolite and - particularly as filling in the cavities - chalcedony, but chalcedony needles also occur in the matrix. The principal secondary mineral is again montmorillonite.

The areas under the peaks in the X-ray diagram revealed the following:

Quartz Plagioclase Pyroxene Zeolite ". Montmorillonite ".

area

700 mm' 870 mm' 102 mm' 60 mm'

1430 mm'

40 BOLETIM OOS SERVICOS DE CEOLOCIA E MINAS

The differences between this sample and No. 174 are the very pronunced

quartz peak at 13.35' (Copper K.,) being d = 3.34 A.. and the marked increase of montmorillonite.

The chemical analysis showed over-saturation (see Table 2) and the Lacroix parametric expression for the rock is 11. 3 (4). 3 (4). 4.

The percentage of crystal water is high and this. together with the high percentage of montmorillonite. indicates a rather advanced state of weathering. More than half of the rock's minerals were found to be secon­dary. namely 54 %. and this classifies the rock as "badly weathered".

Sample No. 172: Residual soil:

The soil is brownish-red and clayey. It covers the lavas with varying thickness but is absent in places.

X-ray analysis showed the presence of quartz which may mostly be blown in. plagioclase. some pyroxene and much montmorillonite. No zeoli­tes could be detected. The evaluation of the areas under the peaks gave:

Quartz ... Plagioclase Pyroxene . Zeolite . Montmorillonite

area 580 mm' 590 mm' 65 mm' o mm'

635 mm'

The percentage of montmorillonite seems to be noticeably lower than in sample No. 173. It must be realised. however. that the X-ray investigation indicated montmorillonite in a rather amorphous state which makes the determination of the area under the peak somewhat uncertain. No kaolinite was found.

The chemical analysis (see Table 2) shows a further increase in the percentage of crystal water but less silica than in sample No. 173. The per­centage of secondary minerals is 62. which is in good agreement with what would be expected for a material of this state of weathering and under the local climatic conditions.

Weathering

The percentage of secondary minerals. the quantity of water and the presence of montmorillonite in all samples show that none of these samples was fresh. The macroscopic appearance of the "freshest" sample. No. 174. had already. in the field. led to classification as "weathered". which was fully confirmed by all investigations.

Both rock samples Nos. 174 and 173 were still hard. but did not ring when struck with a hammer. Both samples had a high percentage of mont-

ESTUDOS. NOTAS E TRABALHOS DOS SERVICOS DE GEOLOGIA E MINAS 41

morillonite. which was their most important secondary mineral. This indi­cated the predominance of chemical alteration of the rocks rather than their physical breakdown; the rocks were decomposing.

The high percentage of water. even in sample No. 174. supports the above statements. A higher percentage of water alone does not necessarily indicate decomposition because it could occur also when a rock disintegrates. The quantity of zeolites. which may be about 10 % in the case of sample No. 174. does not account for nearly 5 % water in this rock. Only about 1 % to 1.5 % crystal water could be assumed to be due to the zeolit,es in sample No. 174. and much less than that in the case of sample No. 173. This leaves much more water in the rock than would be expected for a "fresh" one.

It could be deduced from the stratigraphical position of the two lava flows that the higher one. represented by sample No. 173. had the greater influence on the formation of the residual soil. This is supported. to a certain degree, by the chemical analyses.

Sigvalderson (7) (1959) based investigations on the diffusion of elements during decomposition of a rock on the ratio AI,O,/TiO,. This ratio was chosen because A120 3 is the most resistant oxide normally encoun~ tered in a rock. and because Ti02 in solution is much too unstable to undergo long transport; it will precipitate almost immediately after solution. The ratios of all other oxides to TiO, must also be calculated. e. g. Si02/Ti02•

MgO/Ti02• CaO/TiO, etc. To include all iron in the form of oxides. the calculation must be based on Fe,03+FeO/TiO,. Diffusion curves. as pro­posed by Sigvalderson. could not be drawn usefully for the Moamba lavas because sample No. 173 cannot be regarded as a higher state of weathering of sample No. 174. Such a curve would not allow comparison of the ratio AI,03/TiO, with the ratios of all other oxides to titanium oxide and thus provide SOme information on the diffusion of these elements in the course of weathering.

The general impression gained from separately comparing samples Nos. 173 and 174 with the residual soil (sample No. 172) indicates a definite loss of iron. This is in agreement with the formation of montmoril­lonite. which requires the removal of iron. The quantities of silica and sodium remain more or less unchanged relative to aluminium and there is a definite increase of potassium relative to aluminium as well. The positon with regard to magnesium and calcium is not clear. The residual soil (sam­ple No. 172) has experienced a loss of both of these elements relative to sample No. 174 but a gain relative to sample No. 173.

The position with regard to calcium might be explained from the interaction of two processes: removal of calcium occurred in the early stages of weathering and this was later balanced. and finally surpassed. by secondary precipitation consequent on evaporation.

The diffusion of chemical elements. the high water content and the formation of large quantities of montmorillonite are typical indications of the prevalence of decomposition. The transformation of the montmorillonites

4,2 BOLETIM DOS SERVICOS DE GEOLOGIA E MINAS

from the crystallized to the amorphous structure indicates a high degree of weathering in the residual soil. No kaolinite has, however, been formed which is in agreement with some plagioclase and pyroxene being retained in the residual soil. According to experience, plagioclase and pyroxene should have disappeared completely if the formation of kaolinite from montmorillonite had taken place. Disintegration of the solid rock on the surface is not very important, and might be explained by the effect of running water, in the stream bed causing easy removal of fines while the least disintegrated parts of the rock are left behind.

Climate

It has long been known that climate plays a very important role in weathering and that there is more thorough alteration of rocks in warm, humid climates than in any type of cool or cold ones. The varying engineer­ing perfomance of weathered igneous rocks in South Africa, particularly when used in layers of road foundation, stimulated research on "climate" in regard to weathering.

Almost all individual climatic factors were investigated separately for January and July, as the warmest and coldest months generally, as well as for the full year. Contour maps were drawn for each factor. It was found that only those climatic factors for January which referred to moisture and the annual precipitation gave a contour pattern in agreement with the regional differences of weathering as well as with the engineering perfor­mance of weathered basic igneous rocks. Similar studies, concentrated on these selected factors, were also carried out for the night hours, assuming that the effect of moisture in the process of weathering is stronger the more evaporation is reduced. These investigations led to a zonal subdivision of South Africa, with one narrow zone crossing the country in an approximate north-south direction in agreement with ~he boundaries between types of weathering and observed engineering performance (2), (3). The widely spread occurrence of basic igneous rocks. Karroo dolerites in particular. and the very distinct division of South Africa into summer and winter rainfall, and, in humid and arid areas, created a special problem in this country (8) which proved to be well suited for research on the relation between climate and weathering.

Disintegration is the predominating form of weathering in the dry west of South Africa and decomposition prevails in the more humid east (2). The engineering performance of weathered basic igneous rocks is generally sound where the rocks disintegrate and it is generally unsound where they decompose (3). The boundary between predominating disintegration and decomposition crosses South Africa also in a north-south direct.ion and falls entirely within the above mentioned narrow climatic zone.

The evaluation of these factors showed that the only important ones in defining the climatic border zone, were the potential evaporation for January, calculated from the Meyer formula (9), as applied for shallow

ESTUDOS, NOTAS E TRABALHOS DOS SERVICOS DE GEOLOGIA E MINAS 43

water surfaces (10), and the precipitation in January expressed as the ratio.

(1 )

where E is the potential evaporation, P precIpItation and J January, The other important factor is the annual precipitation which indicates the total quantity of water available as a weathering agent. It is of the greatest impor­tance, however, whether most of the water is precipitated during the warm or cold season of the year, or whether it is evenly distributed. The expression

(2)

where "a" is annual, indicates whether the rain falls mostly in the summer (D > 1), in the winter (D < 1) or whether it is evenly distributed over the year (D - 1),

If D is multiplied by R, a numerical expression is found for the inte­raction of those climatic factors which were considered to be of decisive influence on the form of weathering which will occur. The formula which results from this multiplication reads

(3)

The climatic N-value does not describe a "climate", because the same N-value can be found in different climates, It is simply a numerical expression for the interaction of potential evaporation during the warmest month and the annual precipitation in its distribution over the year in regard to weathering.

The value N = 5 was found to be a critical limit in the generally warm climates. Values of more than 5 indicate conditions which favour disintegration, and values below 5 those where decomposition predominates. Figure 2 gives an overall picture of N-value contours for Africa south of 16' S, and the dotted line indicates the boundary between the prevalence of disintegration to the west and decomposition to the east in the Republic of South Africa. This line is also the boundary between sound engineering performance of weathered basic igneous rocks in road foundations to the west and unsound performance to the east. The latter statement could apparently be confirmed for Southern Rhodesia, where such rocks give a sound performance in the south and unsound in the north.

Figure 3 shows the N-value contours in the Province of M09ambique. The climatic N-value of the site of the Moamba lavas is almost exactly 3, which is somewhat higher than for the high veld to the west. The value is, however, below 5 and there should thus be a definite prevalence of decom­position, as has shown to be the case.

44 BOLETIM DOS SERVICOS DE GEOLOGIA E MINAS

Conclusions

The basic to intermediate lavas near Moamba weather in a manner to be expected from t.he climatic conditions at the site. The formation of montmorillonite from a decomposing basic rock and the transformation of well defined montmorillonite lattices into an amorphous structur!, and, further. the absence of kaolinite formation. are all in complete agreement with a climatic value of N = 3. A similar climatic condition exists. for instance. in the vicinity of Kroonstad in the Orange Free State. and the general appearance of weathered basic igneous rocks there is similar to that near Moamba. In areas of lower N-values. as on the highveld of the T rans­vaal or in Natal. decomposition is much more pronounced and the depth of weathering is notably greater. The formation of kaolinite attains an increasing importance the more the climatic N-values go below 3. and it becomes the predominating clay mineral if the values are lower than 2.

The N-values refer to an ideal plane. This means that topographical effects have not been taken into account and no correlation for such effects can be proposed at the present stage. It can be stated generally. however. that improving run-off conditions shihs the effect of a given N-value so that the appearance of a weathering rock may correspond t.o that of a higher N-value. Such conditions definitely exist at the site of the Moamba lava. and this is probably one of the reasons for the rather solid badly weathered rock.

All three types of material represented by the three samples. Nos. 174. 173 and 172. must be considered unsound from the point of view of a road materials engineer. The percentage of secondary minerals even in sample No. 174 is too high for an area with a climatic value of N = 3. where the percentage of secondary minerals should not be higher than 22 %. Moreover. the high percentage of zeolites would also make the fresh rocks rather unsound. because rapid breakdown of these minerals must be expected. followed subsequently by rapid disintegration of the whole rock if such rock were used for any construction purpose.

Acknowledgments

The authors are indebted to Or. A. de F. Nunes of the Geological Survey of the Province of Mo~ambique for hi. help in collecting the samples near Moamba and the Meteorological Service of the Province of Mo~am­bique for the very valuable climatic information. They are further indebted to the National Chemical Research Laboratory of the South African Council for Scientific and Industrial Research who carried out the chemical analyses.

This paper is published by permission of the South African Council for Scientific and Industrial Research. Pretoria.

..... o

8 Sampling site of basic lava.

I Sampling site of intermediate lava.

S Sampling site of residual soil.

FIG.1. SAMPLING SITE OF MOAMBA LAVA.

r,j-·''-·'" ,_._.,._._. __ '_' __ '_'_"'" ,._. __ . __ .,.'

\ "

".

'. - ::.::.~- - --:~" - ,,,,"!".' //

j-'-'- \ .... // 4~'/ " w"".;~

"

Go~wc l·7.

I 2Q-+---------\

""1 \

".-1-- I I I

W;"dn~. \ IO~l!. \ \ ~ \ '0_.

".--!-----!---1

<0'

28" \~

\ I ,

,,' \ '\

.Gl>on~; ,.,

.V;etori~w •• 1 IS-'

\.

, -\,.PI~':'i'~.S"t.i''''' Sho~";

i Kefi Gwond,,5-9

\.6~

M<>hc1oprc •

" / i

?rd",;o 2-4 •

• Johoon .. bu'!l

"

~."'<"O.t!e. ,., lQdy,,,,lto

. __ ._ ......... 2'2 •

.-" / ~ / L""' 2? "\

I (l) \ I! if -;:-~ i

.Gfeylown

" DurbQni 1-3,

I

'C~I"'Sl tu","

"

4 ,

~ .. ,.

/ j

"

".

" .~./

.I

\!

Alto ,.,.".;<"~~

I 0-9 • .E"~s' ,. ,

PcbQne ,.,

'Anion; • <no, ,.,

) J la'

'-i---i------f-".

jT~~ 22"

o ~

LEGEND

4

5 -- Lines of equal climatic N-values

6

.GREYTOWN Place with N-value ,. 5

H,L N-value "High"(H) and "Low"(L)

00000 w€nthering and performance bound­ary_

',~

wo

\ I "-''' 320 I I 1'\ Gfccf"-Re;net,

• B""ufo,t~~:s! • FIG.2. CONTOUR MAP OF CLIMATIC

N - VALUES FOR AFRICA SOUTH OF 16° S I 'B,.

I ~ w \ 2

SCALE: 13,500,000 ,r I~ N' ". w ,,' M'

'·'~''''''''''''·''''N%-

/." f '\ Zumbo 2 'le 1·3

i . L._. '"

'''"-

3

\ ..... -. Sipolilo

• 2'3 Mt. Darwin • 2·7 "-. -]

<;' Sinoia .Bindura Mtoko \

• 2·6 ~ 2-4 .~3..C-·1_-J....._ ~ew Years Gift

Salisbury. 3·2 i 2·5 Morandollos

• ·Beotrice 2'8

2·0

Premier Estate ~\I -,

• 3'1

E nkeldoorn 3·0

Umvuma 5·3

Fart Victoria 4·0 .Bikito

2·2

~Nuanetsi /1.

7·3

-----. - . 5 1'0 7'5'/

\

\ \ I I

4

) L<I?

~Resso//ia ( • Moamba

4 4

~ Tete 4·0

.~. Louren(Oa Marques 2·8

! . I

~.Piet Retief .. . 1·3 I '" .

/ )

( 1 \.

3

{ , ( " . U

Alto MOlocue. I 0·9

• Errego 0'6

----~--------------20'

nculos ·9 -----------1-22 '

ambone 2,0 __ .

2 4'

LEGEND 4 - lines of equal

5 _Climatic N-Values.

6-

.Br~~o Place with N-Volue.

H,L N-Value "High" (H)

and ulowll (L) 2 6' , Sampling site [leor

Moombo.

FIG,3 CONTOUR MAP OF CLIMATIC N-VALUES FOR PART OF THE PROVINCE OF MOCAMBIQUE

ESTuDOS, NOTAS E TRABALHOS DOS SERVICOS DE GEOLOGIA E MINAS 45

REFERENCES

(I) DE ASSUN<;:AO, c. F. T., COELHO, A. V, P. AND DA ROCHA, A. T.

Contribution to the petrology of the Lebombo laoas (Province of Mos:ambique). C, C. T. A. t Southern Regional Committee for Geology, Pretoria. September. 196 J.

(2) WEINERT, H. H.

Climate and weathered Karroo dolerites. Nature, Vc!. 191, No. 4786, pp. 325H329, 196 J.

(3) WEINERT. H, H.

Climate and the potential performance of weathered dole rites in road foundations. Second Southern Afr. Reg, ConL Soil Mech. and Found. Engng .• Lourenl,;o Marques. 1959.

(4) SCOTT. L. E.

Secondary minerals in rock. as a cause for pavement and base failure. Highw. Res. Board, Proc., 34 th Annual Meeting, 1955.

(5) WEINERT. H. H.

Research on road building materials. Construction in Southern Afr.. Val. 5, No. 7. pp. 35·45. 1960,

(6) VON ENGELHARDT. W.

Obet die Moglichl{eit det quantitativen Phasenanalyse von Tonen mit Rontgens­trahlen. Zeitschr. Kristallogr .. Vo!. 106. pp. 430-459. 1954.

(7) SIGVALDERSON, G. E.

Gesteinszersetzung durch postvul~ani8che Ak,tivWit in Island. Beitr. Miner. und Petrogr .. Vo!. 6, No. 6. 1959.

(8) WEINERT. H. H.

Weathered dole rites in road foundations. a problem unique to South Africa. Construction in Southern Afr .• Vo!. 6, No. 5, 1961.

(9) MEYER, A. F.

CompuUng run-off from rainfall and other physical data. Trans. A. S. C. E .. Vol. 79, pp. 1056·1224, 1915.

(10) L1NSLEY. R. K., KOHLER, M. A. AND PAULHUS, ). L. H,

Applied Hydrology. McGraw-Hill Book Co., Inc., 1949, p. 1.68.