GEOLOGY. 227 



of the Colorado River. 4. The Anotolo limestone is of oolitic age 

 and contains Ceromya similis, Trigonia costata, and other charac- 

 teristic fossils, reminding one strongly of British forms. 5. Ade- 

 grat sandstone, a massive formation, occupying a very extensive 

 area in nortlrern Tigre, and perhaps representing the coal-bearing 

 rocks known to exist north-west of Lake Dembea, but yielding no 

 fossils. 6. Metamorphic rocks of various mineral character, with 

 a general north and south strike due to pre-existing cleavage. 

 Quarterly Journal of Science, July, 1870. 



DIAMOND FIELDS OF SOUTH AFRICA. 



In relation to the geology of the diamond fields of South Africa, 

 which are coming so extensively into notice, Dr. John Shaw 

 says : 



" In July of this year I made considerable observations in the 

 Vaal Valley, which show that the rocks are chiefly trappean, 

 metamorphic, and conglomerate in character. I detected no pure 

 granite formations, but syenite is, however, developed extensive- 

 ly, and seems to be the base of the whole system of rocks at 

 Klipdrift. A very singular rock appears in the shape of isolated 

 boulders on the summits of the Kopjes, and especial!} 7 of the cel- 

 ebrated Aid Kopje. This I take to be graphic granite (binary 

 granite), or what Dana would call ' granilite,' consisting solely of 

 quartz and large crystals of felspar. 



"Above the syenite is a trap-conglomerate in some places, in 

 others are amygdaloids, and protruding through these again ba- 

 salt, assuming eveiywhere the hexagonal structure, and arising 

 in some places into insulated and compacted columns. 



" In some of the Kopjes there are remains of stratified rocks, 

 clay schists, sandstone, chalk (or something very like it), which 

 are evidently the last vestiges of a vast series of sedimentary 

 strata, which formerly covered the whole present contour, but 

 which have gradually given way to denudation and cataclysm. 



" Such is the character of the present rock system at Klipdrift, 

 and, with a few additions (mainly superincumbent), of the whole 

 rock series of the Vaal region. 



" On the summits of the Kopjes, and, as a matter of course, in 

 the crevices between the basaltic boulders, is an alluvial gravel. 

 In this are found diamonds, and on the surface some have* been 

 found, indicators of the wealth beneath. The pebbles of sand- 

 stone, quartzite, crystalline sandstone, granite, clay-slate, agate, 

 tourmaline, iron pyrites, garnet, garnet spinel, etc., which com- 

 pose this alluvium, are all roundedly polished and water-worn, 

 and are imbedded at Klipdrift in a brownish fatty earth. 



"The question arises, Is this alluvium of recent or ancient for- 

 mation ? Did the majority of the pebbles exist in the form of a 

 conglomerate, aggregated from the alluvium of a former age ? 

 Or have the Kopjes at no very late period been the bed of the 

 river ? 



" It is my opinion that the water-worn gravel has been under 



