Petrography of Rocks. 273 



before stated, the original character of the small amount of interstitial 

 matter is indeterminable, but probably little if any glass was origin- 

 ally present. 



Amygdaloidal vesicles seem to be more common near the margin 

 than in the centre of the mass. They are filled with concentric layers 

 of chlorite, calcite, quartz, and chalcedonic silica, in indeterminable 

 order. 



This is apparently the rock which was determined by Stelzner as 

 " olivine-diabase," but I have been unable to find any olivine or any 

 mineral which can be supposed to represent olivine in the specimens 

 at my disposal. One or two of these also show a fair amount of 

 quartz, and it is unlikely that olivine would occur in such a rock. 

 Large masses of sub-basic and basic rock of this type often show a 

 good deal of variation in silica percentage, and a small increase in 

 this respect is sufficient to prevent the formation of orthosilicates of 

 the olivine group. The absence of the mineral in a few slices only 

 is therefore inconclusive. It must be confessed, how^ever, that the 

 general characters of the rock suggest a distinctly more acid type than 

 that to which continental petrologists are in the habit of applying the 

 terms olivine-diabase and melaphyre. The latter, indeed, is defined 

 by Rosenbusch as more or less equivalent to his labradorite-porphyrite, 

 which is an essentially basic rock. However, the materiaj in our 

 possession is insufficient to fully decide this question. It can only be 

 stated provisionally, and subject to modification, that the rock appears 

 to be an intermediate lava, with affinities to the hypabyssal quartz- 

 dolerites. However, it is probably more acid than the majority of 

 these, and cannot strictly be referred to any accepted rock type. So 

 far as the feldspathic constituent is concerned, the rock shows some 

 affinity to the mugearites of Skye described by Harker (*), which are 

 essentially oligoclase-rocks, with subordinate orthoclase. However, 

 in these Scotch rocks augite is quite subordinate to olivine and iron 

 ores, whereas in the Kimberley rock olivine is perhaps absent, and, 

 at any rate, not abundant, while augite is the dominant ferro- 

 magnesian mineral. 



Below the " melaphyre," at a depth of 740 feet begins another 

 series of sedimentary rocks, which has a total thickness of some 660 

 feet in the Kimberley Mine, and over 700 feet at De Beers. The 

 section given by Gardner Williams shows 400 feet of quartzite above, 

 with 260 feet of shale below, but our specimens from various depths 

 indicate an alternation of these two types of sediment, as shown in 

 the table (fig. A.), (206b., 206k., 2o6g. , 207, 207b.). 



A specimen of the quartzite from a depth of 1000 feet (206b.) 

 consisted originally of well-rounded grains of quartz and feldspar, 

 together with compound grains or small pebbles of various fine- 

 textured, siliceous rocks. The feldspar is mostly orthoclase and 

 microcline, and this and the rock fragments together make up at a 

 rough estimate 10 per cent, of the whole. There are also a few 

 crystals of a deep reddish yellow, isotropic mineral, which is probably 



(*) Tertiary Igneous Rocks of Skye. Mem. Geol. Survey, 1904, p. 264. 



18 



