Petrography of Rocks. 



A. THE ACID VOLCANIC SERIES. 



Below the sedimentary rocks last described there is found another 

 great series of igneous rocks, which, though much decomposed, appear 

 to be on the whole of a distinctly acid character. The examination 

 -of numerous specimens shows certain points of resemblance in almost 

 all cases, but still it is possible to distinguish two more or less well- 

 defined types, which, as a matter of convenience, are here described 

 as amygdaloid and quartz porphyry respectively. The differences are 

 probably more apparent than real, and are more conspicuous in hand 

 specimens than in thin slices. Speaking broadly, however, we may 

 say that the amygdaloidal rocks contain few phenocrysts of quartz, 

 while in the non-vesicular types this mineral is abundant in the 

 porphyritic condition. 



Our collection contains a good many specimens of these rocks 

 from different depths, and from a consideration of these it is evident 

 that the amygdaloid and the quartz porphyry occur in alternating 

 beds or flows. The total thickness of the whole group is very great, 

 amounting at the Kimberley Mine to about 1070 feet. At the De 

 Beers Mine it is less, since the surface of the granite is here about 600 

 feet higher. 



Although these rocks are here treated of under two separate 

 headings, it must be clearly understood that there is every transition 

 between them, and some of the specimens are so much decomposed 

 that their original character is very doubtful. 



(a) The Amygdaloid. 



In a hand-specimen (208a, 208b), this is a pale, greyish green 

 rock, fine in texture, and with a somewhat streaky appearance. It 

 contains very abundant rounded or oval vesicles, filled with dark grey 

 or white minerals. Examinations of slices show that the rock has 

 undergone so much alteration as to make determination of its original 

 character difficult, if not impossible. 



It contains abundant porphyritic crystals, which have the charac- 

 teristic form of feldspars, and are now represented bv pseudomorphs 

 of somewhat varying character. Many of them are more or less 

 silicified, consisting of an aggregate of quartz, white mica, and some 

 pale green chloritic substance, which shows very weak birefringence. 

 The flakes of white mica usually show a distinct arrangement along 

 two sets of planes, more or less at right angles, presumably the 

 cleavage planes of the original feldspars. It is quite impossible to 

 determine the original character of the feldspar, since no part of 

 the original substance seems to remain. 



The ground mass is of a peculiar character, consisting of a very 

 line-textured aggregate of quartz and feldspar : it shows the peculiar 

 patchv structure known as micropoecilitic, which is defined bv Harker 

 as follows* : — " This consists of minute feldspar crystals with no 

 orderly arrangement, enclosed in little ovoid or irregular areas of 



* Petrology, 3rd Ed., 1902, p. 163. 



