Ch. IV.] CONSOLIDATION OF STRATA. 35 



stony near Kelloway. In this district there are numerous fossil shells 

 which have decomposed, having for the most part left only their casts. 

 The calcareous matter hence derived has evidently served, at some former 

 period, as a cement to the siliceous grains of sand, and thus a solid sand- 

 stone has been produced. If we take fragments of many other argilla- 

 ceous grits, retaining the casts of shells, and plunge them imto dilute 

 muriatic or other acid, we see them immediately changed into common 

 sand and mud ; the cement of lime derived from the shells, having been 

 dissolved by the acid. 



Traces of impressions and casts are often extremely faint. In some 

 loose sands of recent date we meet with shells in so advanced a stage of 

 decomposition as to crumble into powder when touched. It is clear that 

 water percolating such strata may soon remove the calcareous matter of 

 the shell ; and, unless circumstances cause the carbonate of lime to be 

 again deposited, the grains of sand will not be cemented together ; in 

 which case no memorial of the fossil will remain. The absence of or- 

 ganic remains from many aqueous rocks may be thus explained ; but 

 we may presume that in many of them no fossils were ever imbedded, 

 as there are extensive tracts on the bottoms of existing seas even of 

 moderate depth on which no fragment of shell, coral, or other living 

 creature can be detected by dredging. On the other hand, there are 

 depths where the zero of animal life has been approached ; as, for ex- 

 ample, in the Mediterranean, at the depth of about 230 fathoms, accord- 

 ing to the researches of Prof. E. Forbes. In the JEgean Sea a deposit 

 of yellowish mud of a very uniform character, and closely resembling 

 chalk, is going on in regions below 230 fathoms, and this formation 

 must be wholly devoid of organic remains.* 



In what manner silex and carbonate of lime may become widely dif- 

 fused in small quantities through the waters which permeate the earth's 

 crust will be spoken of presently, when the petrifaction of fossil bodies 

 is considered ; but I may remark here that such waters are always 

 passing in the case of thermal springs from hotter to colder parts of the 

 interior of the earth ; and as often as the temperature of the solvent is 

 lowered, mineral matter has a tendency to separate from it and solidify. 

 Thus a stony cement is often supplied to sand, pebbles, or any fragment- 

 ary mixture. In some conglomerates, like the pudding-stone of Hertford- 

 shire (a Lower Eocene deposit), pebbles of flint and grains of 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 con- 

 firm 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 ; and these, when once 



* Report Brit. Ass. 1843, p. 178. 



