ROCK-FOLDS 



By PROF. ERNEST H. L. SCHWARZ, A.R.C.S., F.G.S. 



Rhodes University College, Grahamstown, South Africa 



A bed of sandstone or shale buried deep in the earth's crust 

 becomes as adaptable to mountain-building forces as if it were 

 so much putty, no matter how hard and intractable it may 

 appear when exposed on the surface after the overlying material 

 has been carried away by denudation. This adaptability is not 

 inherent in the rock itself, in the same way as plasticity is a 

 character of putty, elasticity that of india-rubber, and so forth, 

 but is due to the solvent action of water, which at moderate 

 temperatures and pressures is able to carry away portions that 

 are under compression and to deposit the substance again in 

 positions of tension. The action is well illustrated in the 

 rounded limestone pebbles of the Nagelfluhe, on the foot-hills 

 of the Jura, where the pebbles may be seen flattened ; or, again, 

 in the tiny ovoid grains of oolitic limestones which have been 

 subjected to pressure, as in those of Ilfracombe. In the case of 

 quartz and silicates which are not soluble in water at the surface 

 of the earth, the case would appear to be more difficult of 

 explanation, for we find that the originally water-worn pebbles 

 of the older conglomerates are not only flattened, but are often 

 pulled out into long fingers, as we may see in many of the hand- 

 specimens of the Cango Conglomerate in Oudtshoorn, South 

 Africa; in North American rocks the same feature has been 

 beautifully illustrated by Mr. McCallie. Experimental evidence 

 in favour of this solution of silicates by pure water has been 

 afforded by Barus ; on subjecting water contained in capillary 

 tubes to considerable pressure and a temperature of 185 C, the 

 volume of the water apparently decreased, but it was found that 

 the loss was accounted for by the fact that some of the glass had 

 been dissolved and redeposited in a crystalline state, the latter 

 being a state in which, owing to the closer packing of the 

 molecules, the material is denser and occupies less space than 



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