120 K H. L. Schwarz—The Thickness of the Ice-Cap. 



below 1300°, and, from Joly's experiments, fuses at 1200° or even 

 lower. But tridymite is stable below 1800°, and only fuses when 

 that temperature is exceeded. Those familiar with the phase rule 

 will i-emember instances of a substance having two solid phases each 

 with a different fusion-point. It should also be noted that it was by 

 rapidly heating the quartz up to and beyond the transformation point 

 that the higher fusion-point was realized. 



The vapour pressure would have some effect in modifying the 

 temperature of fusion of these two phases of silica, and though, 

 owing to the absence of data, this could not be profitably discussed 

 here, yet it is a factor that should be considered when dealing with 

 a cooling fused rock-mass. 



The significance of the possibility of silica appearing in either of 

 its two solid phases cannot be overlooked owing to its obvious 

 bearing on the question of the thermal conditions attending the 

 consolidation of the igneous rocks. Treating the cooling mass as 

 a case of reciprocal solution, it would appear that the temperature at 

 which silica may be thrown out of solution is a critical point of some 

 importance. If it happens to be above 1300° C., and neglecting for 

 the present the effect of vapour pressure, then the silica would 

 appear in the form of tridymite. If the temperature of the masS 

 happened to be below 1300° C. when silica was thrown out of 

 solution, then quartz would appear. 



Quartz is generally one of the later minerals to appear in a cooling 

 rock-mass, therefore its predominance in igneous rocks as a phase 

 of silica is not surprising. The comparative rarity of the tridymite 

 phase would also be explained. If the correctness of the writer's con- 

 clusions be admitted, then we are able to fix within moderate limits 

 the temperature of certain critical points in the cooling curves of the 

 igneous rocks. 



There is, of course, just a possibility that tridymite is really 

 metastable below the transformation point, but that it requires 

 a long period of time before passing into the more stable form. 

 In that ca?e the geological age of the tridymite -bearing rocks 

 becomes a factor of some importance. 



V. — The Thickness of the Ice-Cap in the various Glacial 



Periods. 



By Ernst H. L. Schwarz, A.E.C.S., F.G.S., 



Rhodes University College, Grahamstown, South Africa. 



IN estimating the maximum load which pressed upon the northern 

 type of Glacial (Dwyka) Conglomerate in Prieska, Cape Colony, 

 I assumed that the calculations of Sir Wy ville Thomson and Bernacci 

 were correct, and that the greatest column of ice that could exist 

 on the earth's surface was from 1,400 to 1,600 feet high.' This 

 limit, however, is by no means accepted by European glacialists, 

 who, though they do not go as far as Dr. Croll in assuming 



1 "An Unrecognized Agent in the Deformation of Eocks " : Trans. S. African 

 Phil. Soc, vol. xiv (1903), p. 400. 



