52 



LYELL'S ELEMENTS OF GEOLOGY. 



Consolidation of Strata. 



the grains of sand will not be cemented together ; in which case 

 no memorial of the fossil will remain. The absence of organic 

 remains from many aqueous rocks may be thus explained. 



In some conglomerates, like the puddingstone of Hertfordshire, 

 flinty pebbles and sand are united by a siliceous cement so firmly, 

 that if a block be fractured the rent passes as readily through 

 the pebbles as through the cement. 



It is probable that many strata became solid at the time when 

 they emerged from the waters in which they were deposited, and 

 when they first formed a part of the dry land. A well-known 

 fact seems to confirm this idea ; by far the greater number of the 

 stones used for building and road-making are much softer when 

 first taken from the quarry than after they have been long 

 exposed to the air. Hence it is found desirable to shape the 

 stones which are to be used in architecture while they are yet 

 soft and wet, and while they contain their " quarry- water," as it 

 is called ; also to break up stone intended for roads when soft, 

 and then leave it to dry in the air for months that it may harden. 

 Such induration may perhaps be accounted for by supposing the 

 water, which penetrates the minutest pores of rocks, to deposit 

 on evaporation carbonate of lime, iron, silex, and other minerals 

 previously held in solution. These particles, on crystallizing, 

 would not only be deprived themselves of freedom of motion, 

 but would also bind together other portions of the rock which 

 before were loosely aggregated. On the same principle wet sand 

 and mud become as hard as stone when frozen ; because one 

 ingredient of the mass, namely, the water, has crystallized, so as 

 to hold firmly together all the separate particles of which the 

 loose mud and sand were composed. 



Dr. MacCulloch mentions a sandstone in Sky, which may be 

 moulded like dough when first found ; and another from China, 

 which is compressible by the hand when immersed in water. But 

 it is not merely these compounds which readily admit water to 

 penetrate into them ; some simple minerals, says the same wri- 

 ter, which are rigid and as hard as glass in our cabinets, are 

 often flexible and soft in their native beds ; this is the case with 

 asbestos, sahlite, tremolite, and calcedony, and it is reported also 

 to happen in the case of the beryl.* 



The marl recently deposited at the bottom of Lake Superior, 

 in North America, is soft, and often filled with fresh- water shells ; 

 but if a piece be taken up and dried, it becomes so hard that it 

 can only be broken by a smart blow of the hammer. If the lake 

 therefore was drained, such a deposit would be found to consist 



Dr. MacCulloch, Syst. of Geol. vol. i. p. 123. 



