THOSrHATES IN THE CAMBRIAN ROCKS. 375 



composition changed and altered by the breaking- up of the rocks 

 through which they were forced, and by the absorption of portions 

 of these into their own substance. The phosphate and carbonate of 

 lime in these masses may therefore have been obtained out of the rocks 

 through which they were injected in the molten state; and these in- 

 gredients, in the condition in which they are found, may not have 

 existed (or, at least, not in so large a proportion) in the intrusive 

 mass prior to its meeting these sedimentary beds. This, of course, 

 will not and is not intended to explain the origin of all the phosphoric 

 acid in eruptive rocks ; but it may nevertheless tend to explain the 

 great preponderance present in some of the eruptive masses and the 

 almost entire absence of it in others. In those passing through beds 

 rich in organic materials we may naturally expect a preponderance ; 

 whilst in those dykes which, so far as we are able to judge, appear 

 only to have met with barren strata, little should be expected • and 

 I believe this is very much what is found to be the case. 



The general conclusions which we seem to arrive at by these 

 examinations and experiments are : — (1st) that animal life has been 

 the chief means by which phosphate of lime has been produced from 

 the very earliest time of which we are able to trace the record up to 

 the present ; that though all animal and vegetable life in the Cam- 

 brian and Lower Silurian «eas contributed to its production, yet of 

 the forms of life which are known to have lived during those epochs 

 the Crustacea seem to have had by far the greatest share in its pro- 

 duction, and of the Crustacea the Trilobites more particularly. Not 

 only does it occur more abundantly wherever the Trilobites are pre- 

 sent, but our analyses have proved that their shell, when preserved, 

 contains an extraordinary amount of this ingredient, nearly one half 

 being phosphate. The largest proportion known in recent Crustacea 

 is that made out by Mr. Hudleston in the shell of the lobster — that is, 

 7*12 per cent of phosphate of lime ; it is probable therefore that the 

 Trilobites had a much larger amount in their shell than is to be found 

 in recent Crustacea, where its place seems to be taken by carbonate 

 of lime. We can hardly suppose, however, that the whole of the 

 phosphate of lime which we now find to be present in the fossil 

 occurred also in the natural shell ; for during the process of decom- 

 position of the animal matter, in which doubtless P 2 O g occurred also 

 in considerable quantity, the shell may have been made to undergo 

 an important change, and a substitution have taken place. Still the 

 great excess in the fossil compared with what is found in the matrix 

 or generally throughout the rock leads one to suppose, especially when 

 we take into consideration its comparative absence in the other fossil 

 remains, that the natural shell did contain phosphate of lime in an 

 unusual quantity. I am inclined to think that the shell of the Trilo- 

 bite was of a more horny texture than that of the recent Crustacea, 

 being somewhat of a character intermediate between the calcareous 

 shell of these and the so-called horny shell of Lingula. Therefore, 

 though it is useful to know the amount which by the analogy of 

 recent Crustacea, the Trilobites may have been able to supply to 

 the rocks, yet it would be most hazardous to rely on this test as in 



