AXD ASSOCIATED KOCKS OF THE SOUTHERN TRANSVAAL. 427 



Main Reef Series. In the Vogelstruis Mine (see Map, PI. XI.) a 

 diamond driU cut into a dyke of diorite. The dyke, however, 

 proved to be nearly vertical, so that boring was stopped before the 

 thickness of the dyke could be ascertained. In the eastern portion 

 of the same mine a nearty vertical dyke about 30 feet thick was 

 drilled through. The composition of this diorite is unlike that of 

 any other igneous rock in the Transvaal. A large proportion of the 

 felspar is decomposed, and the lime has separated out in the form 

 of calcium carbonate. Prom the evidence of the surface dumps of 

 the Percy and Jumpers Mines it appears that a dyke of the same 

 material has been passed through there. 



With the exception of the thin grey ash-beds, already mentioned 

 as occurring near Germiston and Boksburg, nothing else of a vol- 

 canic or igneous nature is associated with, or interferes with the 

 Iteef Series. 



In the western portion of the Princess Mine a dyke filled with 

 blue clay, about 40 feet thick, runs nearly vertically across the beds. 

 The clay is very much compressed, and seems to have filled up an 

 open fault, the sides of which afterwards closed in. So far as seen 

 there appears no tendency in the dyke to narrow as it descends. 



None of the southernmost reefs are pierced, so far as I am 

 aware, by any igneous rock, though they are in close proximity to 

 a thick mass of basalt. 



In the quartzites and red shales lying below the gold-bearing 

 series there are abundant intrusions of igneous rock. In the second 

 valley north of Auckland Park (see fig. 5, p. 421) a sheet of very 

 fresh-looking diorite, with the crystals of hornblende but little 

 decomposed, pierces the beds, sending veins into both the quartzites 

 and the shales ; and in the northernmost, or fourth, ridge of quartzite, 

 previously described, thin sheets and small veins find their way 

 along the joints and bedding-planes. 



Igneous material, but of a totally distinct type, occurs locally at 

 the surface and intrudes into the conglomerates and other beds con- 

 stituting the hill behind Jeppe's Township (see fig. 4, p. 419). The 

 rock is a quartz-amygdaloid lava of an intermediate character, con- 

 taining porphyritic crystals of a dirty-white felspar. This rock also 

 fills up a large portion of the valley east of Doornfontein. 



Both these types of igneous rocks very strongly resemble in their 

 general characters the dominant igneous material of the Southern 

 Transvaal. They are but little altered, and have not been afiected 

 by the movements that have tilted, shifted, and often ground up the 

 associated strata. 



It is certain therefore that the upheaval, dislocation, and folding 

 of the "Witwatersrandt strata occurred before the grand eruptions or 

 intrusions of igneous rocks, and that these eruptions and intrusions 

 took place at least at two distinct periods. It seems equally certain 

 that it was not the granitic base which moved, but that the over- 

 lying rocks were thrust over an underlying and much older crystal- 

 line series. 



