Petrography of Rocks. 277 



The extinction angle of tlie plagioclase, measured on the albite 

 twin lamellae, ranges up to about 22^". Its index of refraction does 

 not differ much from that of orthociase, so that it is probably albite. 



The rock contains no phenocrysts of ferromagnesian minerals, 

 but there are a very few small irregular crystals of a partially- 

 decomposed iron ore. 



The groundmass is microcrystalline, consisting of quartz, 

 feldspar, and a good deal of pale green chlorite, its general appear- 

 ance is distinctly suggestive of silicification, as it is micropoecilitic, 

 with a tendency in places to a microspherulitic structure. In some 

 forms vesicles are absent, in other quartz-bearing specimens a few 

 vesicles occur, and, as before stated, every transition can be traced 

 to the highly-amygdaloidal type. 



The foregoing description is principally taken from the freshest 

 specimen, which comes from a depth of 1840 feet in the Kimberley 

 Mine (208b), and probably approximates most closely to the original 

 character of the rock. Other specimens are much more altered, and 

 in particular many have undergone a high degree of silicification, so 

 that both phenocrysts, vesicles and ground mass are converted into a 

 mosaic of clear, granular quartz. A good deal of calcite is also often 

 present. 



The rocks described in the two preceding sections must be 

 regarded as forming one continuous series of a volcanic nature, but 

 it is not clear whether some of the non-vesicular types are true lava 

 flows. It is possible that some of them may be of the nature of sills 

 or other intrusive masses injected into a previously -existing series of 

 lavas. There is not sufficient evidence at our disposal to decide this 

 point, which can only be settled by an examination of the sections 

 on the spot. 



B. BASIC DYKES. 



At various depths in the acid volcanic series there are found rocks 

 of a much more basic character, which are referred to in the field 

 notes supplied as dykes. These specimens come from the Kimberley 

 Mine, at depths of 1520 feet, 1840 feet, and 2160 feet respectively. 

 They differ very markedly from the surrounding acid volcanic rocks, 

 and present some special points of interest. The three specimens vary 

 to a certain extent among themselves, but they have sufificient features 

 in common to suggest a genetic connection. 



The freshest example comes from a depth of 1520 feet (222), and 

 is a rock of peculiar nature. Its macroscopic appearance is that of 

 a slightly porphyritic, dark-coloured rock of distinctly basic appear- 

 ance, with no visible feldspar. Under the miscroscope it is seen to 

 consist almost entirely of colourless pyroxene and a green chloritic 

 mineral, with only a little interstitial feldspar, and the merest trace 

 of quartz. 



The ferromagnesian mineral clearly occurs in two generations, 

 both as phenocrysts, and constituting the bulk of the ground mass. 

 The crystals of the first generation are idiomorphic, usually in very 

 well- formed, eight-sided crystals, which often show polysynthetic and 



