ROCK-FOLDS 57i 



imagine that this top fractured segment of rock is to account for 

 the transmission of the lateral pressure which buckled up the 

 mountains, it must have been thrust several miles distance over 

 the underlying strata. The actual mileage does not concern us 

 here, but the fact remains that rocks, fissured and structurally 

 weak as we find them, must, on such a supposition, have been 

 bodily transported, despite their weight, and have carried with 

 them a battering power sufficient to crumple up rocks as resist- 

 ant, at any rate, as themselves. It is evident, therefore, that 

 neither in the zone of fracture nor in the zone of flow can lateral 

 thrust be transmitted for any distance in the earth's crust. 



The difficulty is beautifully illustrated by the behaviour of 

 the sheet of lead in the bottom of Mr. Mellard Reade's famous 

 sink. Lead is too pliant to transmit pressure, yet at one end 

 of the sink, in response to a disturbing force represented by 

 the alternate cooling and heating effect from the water let in 

 by the hot and cold water-taps, there arose a well-defined 

 anticlinal ridge at the other end of the sink. The explanation 

 of this, I believe, lies in the expansion and contraction of the 

 metal, which sets up compressional waves ; these pass through 

 the main body of the lead sheet, producing very little effect 

 until they are retarded by the fixture of the sheet to the sides 

 of the sink. The sum of these constantly repeated waves 

 produces a shift in the particles of which the lead is composed, 

 leading to permanent distortion ; the return wave, which would 

 have shifted the particle back to its original position, being 

 damped out by the sides. 



If we now fix our attention on a grain, say, in a sandstone 

 which is undergoing elevation in a mountain chain, and if we 

 suppose that there is a constant propagation of earthquake 

 waves from north to south, and that these waves are retarded 

 by some means, let us follow what would happen. The grain 

 lies surrounded with water, which is ready to dissolve a portion 

 of its substance directly some differential stress acts upon 

 the particle. The wave comes along, drags at the obstruction, 

 and produces a tendency to distortion. The sand grain under 

 such circumstances would have a little of its substance dissolved 

 from the area in which the pressure momentarily acts, and 

 this would be shifted and redeposited in the positions of tension. 

 For any one wave the substance affected might be very small 

 indeed, but the aggregate in the centuries would allow the 



