CONSOLIDATION BY HEAT 1 83 



hard and firm, like the latter. This is not difficult, for we have 

 abundant evidence to prove that such consolidation actually does 

 take place, and in a variety of ways. 



(1) Consolidation by Weight of Sediment. — When deposited 

 on a sinking sea-bottom, sediments often accumulate in masses 

 of great thickness, and in such cases the lower portions must 

 tend to consolidate from the weight of the overhang masses. Of 

 course, such a process cannot be directly observed in modern 

 accumulations, because only the surface of them is accessible, but 

 from the analogy of observed facts we may safely infer that this 

 weight is not without effect. 



(2) Consolidation by Cement. — Sediment is often penetrated by 

 percolating waters, which carry in solution various cementing sub- 

 stances, such as Si0 2 , CaC0 3 , FeC0 3 , etc., and the deposition of 

 these materials in the interstices of the loose sediment will bind 

 the particles into a firm rock. This process we have already had 

 occasion to observe in several instances, as in the coral reefs, the 

 drift-sand rock of Bermuda, the modern sandstones on the coast 

 of Asia Minor, and many others. In all of these cases the cement- 

 ing substance is CaCO a , but other modern rocks are known in 

 which Fe 2 3 , formed by the oxidation of FeCO s , plays the same 

 role. Both of these substances are very common as cements 

 among the ancient rocks. A cementing effect may also be pro- 

 duced by reactions within the mass of the sediment itself, as is 

 seen in the solidification of many volcanic ashes mingled with 

 water to form tuffs. 



(3) Consolidation through Heat. — This may be local, as in the 

 neighbourhood of volcanoes, or general and due to the internal 

 heat of the earth. For sediment to reach great thickness it must 

 subside, and this subsidence brings the lower parts of the mass 

 deep down into the crust, where they are invaded by the earth's 

 interior heat, and baked as bricks are burnt in a kiln. This pro- 

 cess is likewise one which cannot be directly observed, but the 

 effects of molten lava upon loose sediments may be watched, 

 and the consolidating power of heat has been tested experi- 

 mentally. 



