570 SCIENCE PROGRESS 



in the glassy condition. The rate of solution and redeposition 

 was found to be half an hour for a given volume of water to 

 dissolve and redeposit an equal bulk of glass, at the particular 

 temperature of the experiment, namely, 185 C. If we now 

 calculate supposing that the natural minerals in a rock are 

 ten times as resistant to solution as the glass in the capillary 

 tubes, and that the water held in the pores of the rock is one- 

 tenth per cent, of the whole bulk, at 185° C. the rock could be 

 wholly dissolved and redeposited in 5,000 hours, or considerably 

 less than a year. As a matter of fact, however, most crystalline 

 rocks, on analysis, yield more than *i per cent, of water, some 

 schists and gneisses holding as much as 2 per cent, by weight 

 or 5 per cent, by volume, which means that such rocks could be 

 wholly dissolved and recrystallised every 100 hours or, say, 

 once a week, at a depth at which a temperature of 185 C. exists, 

 roughly some five miles beneath the surface. 



In a homogeneous bed of sandstone at five miles depth within 

 the earth's crust, a pressure acting upon it for a few days would 

 fail to be transmitted, because every grain would allow itself 

 to be transformed into disc, flattened normal to the pressure. 

 Such an extreme case does not often happen, because the 

 weight of the rocks above prevents the vertical flow which 

 such a horizontal contraction would necessitate, but so much 

 accommodation would be provided for in the bed of sandstone 

 that a force striving to be transmitted through it would soon be 

 lost. Supposing the rocks immediately above and below the 

 sandstone were perfectly rigid and water-tight, then the force to 

 be transmitted through the sandstone would act as if it were 

 applied to a liquid ; but in nature rocks are not water-tight, and 

 they are so far from being rigid that, apart from their power 

 of being dissolved and redeposited, they are always ready to 

 give way to any pressure owing to the constant tremors 

 which agitate the earth's crust. If then we recognise that 

 silica and silicates become soluble under pressure force cannot 

 be transmitted through a stratum of rock in the zone in which 

 solution and redeposition can go on. Above this zone the 

 rocks are fractured ; cavities can and do exist, and water no 

 longer permeates only the rock substance, but circulates in the 

 fissures as well. Such material is not a fit one to transmit 

 great and prolonged pressures ; if we look at the folds in any 

 considerable mountain range we shall find that if we are to 



