424 Frof. E. E. L. Sc/iimrz—T/ie Watevherg Sandstone. 



post-Torridon granite, but it is invariably restricted to within a few 

 feet of that margin ; in the older Archaean areas it may cover many 

 hundreds of square miles without a break ; it is regional. The mode 

 of occurrence of the rocks in the Gavarnie and Heas Valleys, described 

 by Mr. Dixon, is characteristic of the regional or Archcean type of 

 ther mo-m et amorphi sm . 



In both types of areas, as we approach the zones of increasing 

 metamorphism, a seriefe of minerals is met with which requires an 

 increasing temperature for their development. The Archsean areas of 

 thermo-metamorphism can be broadly distinguished from those due 

 to post-Torridon intrusions by the far more rapid increase in coarse- 

 ness of texture of the groundmass of the former, (/ well foliated. The 

 difference may be illustrated diagrammatically by arranging a series of 

 vertical and horizontal co-ordinates. In the diagram (p. 423) the 

 minerals a, b, c, d, etc., represent increasing temperature, while 1, 

 2, 3, 4, etc , represent increasing coarseness of texture. The lines 

 X, Y, Z show the difference in texture produced by the older and 

 newer thermal metamorphism. 



EXPLAXATIOX OF PLATE XIX. 



Fig. 1 (No. 190). — Rock-section taken from the east side of the Pau Valley at 



Gavarnie (see text for fuller details). 

 FiG-. 2 (No. 191). — Eock-section from half-way between the River Pinara and Port 



de Bielsa ; a very fine spotted schist or phyllite. 



The two photographs show the difference in size of grain of the quartz -felspar 

 material in (1) the older Archtean rocks, and (2) in any post-Torridonian rocks; 

 the crystalline metamorphism being due in both cases to thermal action. 



VI. — The "Watekberg Sandstone. 



By Professor E. H. L. Schwarz, A.R.C.S., F.G.S. (Rhodes University College, 

 Grahamstown, South Africa). 



rpHE geology of South Africa is once again being brought to the 

 JL notice of European geologists in the able papers by Dr. Voit 

 in the Zeitschrift filr prahtische Geologic. In the first instalment 

 Dr. Voit follows Drs. Hatch & Corstorphine ^ in correlating the 

 "Waterberg Sandstone with the Table Mountain Sandstone, which is 

 a small matter perhaps, but as it would in Europe be equivalent to 

 correlating the Old Red Sandstone with the Torridon Sandstone, 

 it is necessary to state the objections to this correlation and enable 

 geologists to judge for themselves whether it is right or wrong. 



To emphasize the difference between the South African older and 

 younger rocks, which are separated by a great unconformity, and the 

 younger series of which begins in the Palaeozoic with the top of the 

 Silurian or base of the Devonian, I have used the words Palaeo- or 

 Pal-Afric and Neo-Afric as group names. The Table Mountain Sand- 

 stone is at the base of the Neo-Afric group, and the Waterberg 

 Sandstone at the top of the Pal-Afric; the former is a blue, false- 

 bedded sandstone weathering grey, and the latter consists of a series of 

 conglomerates and sandstones characteristically coloured red or buff. 



1 " Geology of South Africa" (London, 1905), p. 179. 



