ANALYSES OE SOME CAMBRIAN ROCKS. 381 



in a volume of geological history to which we have no access. The 

 geological record is like the Sybilline Books : none can say what 

 is the number of the volume, reckoning from the beginning, which 

 we are to assign to that containing the history of the formation at 

 present under consideration. It must, however, be granted that at 

 that particular stage of the earth's history which we call Cambrian 

 a considerable amount of organic progress had already been made, and 

 that abundance of nitrogenous food rich in phosphates was then at 

 the command of the great Trilobites. 



But we do not suppose that Crustacea were the only centres from 

 which phosphatic accumulations were derived at this time a Wher- 

 ever organic matter, containing nitrogen in the proportion usual in 

 protein compounds (albuminous principles), was decaying, whether 

 the organization was simple or complex, animal or vegetable, it is 

 certain that an amount of calcic phosphate would be set free. It. 

 would be useless to quote the numerous analyses bearing on this 

 subject ; but, as a rule to be deduced therefrom, it may be stated that 

 the ash of protein compounds consists for the most part of phos- 

 phates. These being comparatively fixed substances, would tend to 

 accumulate in the various sediments at the bottom of the ocean ; 

 whilst the elements more readily volatilized, the carbon, the 

 hydrogen, the nitrogen, and the oxygen, in various gaseous combina- 

 tions would, during the process of putrefaction, escape, and be held 

 for the most part in solution in the overlying waters. This easily 

 serves to account for the presence of disseminated phosphoric acid, 

 though of course the salts (usually of calcium) of which it forms 

 the acid element undergo endless modifications, according to the 

 solvents and reagents to which they are exposed. Thus it often 

 happens that phosphates are largely accumulated round certain 

 bodies in the various strata; and numerous theories have been 

 brought forward to account for this. Some have thought that a 

 gelatinous phosphate has been precipitated from highly phosphatized 

 waters by ammonia generated through organic putrefaction. This 

 takes place in a soil ; but such action, by rendering the phosphates 

 immediately insoluble, would tend to random precipitation unfavour- 

 able to aggregation round a centre. 



Bearing in mind that chloride of sodium increases the solubility 

 of calcic phosphate in carbonated waters, we may well imagine that 

 an appreciable amount of calcic phosphate occurs in solution in the 

 bottom water of the sea, where the quantity of carbonic acid is 

 notoriously greater than in the upper layers. (Incrustations of steam- 

 boat boilers, taken even from the upper layer, but possibly in shallow 

 places, have yielded 0*04 per cent, of phosphoric anhydride.) Such 

 phosphate would be derived principally from the undeposited residue, 

 the result of putrefaction of nitrogenous bodies, as previously stated. 

 In this condition it would be very apt to combine with any albumi- 

 nous body, more or less buried in the ooze, which was softening 

 previously to decay. Indeed we know that chemical compounds of 

 albumen and of gelatine are made, which contain, in the case of the 

 former, as much as 30 per cent, by weight of calcic phosphate. 



