the Silurian and Cambrian Limestones. 371 



thia element had not been overlooked by chemists, I examined 

 carefully specimens of Iceland spar, arragonite, and calc-spar 

 from Derbyshire and Durham, by the same method as that em- 

 ployed in the above analyses of the limestones, but in no case did 

 I fiiid any trace of phosphoric acid. 



In the specimen of limestone from Krageroe, which, from its 

 occurrence, seems probably to have been deposited fi'om water 

 holding Ume in solution, the amount of phosphoric acid present 

 is accounted for by the immediately surrounding rocks containing 

 phosphate of lime ; but even in this the amount is less than might 

 be expected, and much below the per-centage in the stratified 

 limestones. 



In a single specimen of calc-spar from Naeskiil, near Arendal, 

 did I find a trace of phosphoric acid, but too small to be easily 

 determined ; in this case, however, the calc-spar lined fissures in 

 granite which contained crystallized apatite. 



The calc-sinter deposited from the Carlsbad springs was found 

 by Berzelius to contain a small amount of phosphoric acid, but 

 as these springs break through granite, it is possible that the 

 phosphoric acid present in the sinter may arise from a similar 

 cause. 



The results of these analyses, therefore, seem if anything to 

 strengthen the view that all stratified limestones are the result 

 of the development of organic life. It must, however, be remem- 

 bered that under the term stratified limestone, are not included 

 the irregular deposits of travertine, calc-sinter, &c., which 

 obviously are thrown down from springs surcharged with car- 

 bonate of lime. 



That beds of limestone become less and less numerous as we 

 descend in the geological scale, is easily accounted for, even with- 

 out taking into consideration the comparatively small develop- 

 ment of organic life duinng those early epochs. Whatever view be 

 taken of the origin of limestones, it cannot but be admitted that 

 the lime which constituted them must have been originally in a 

 state of solution, and chemical reasons may be adduced for 

 assuming that in the earlier geological periods a but scanty 

 supply of lime in a soluble state was present. 



The oldest granitic rocks contained but a very small amount 

 of lime, and even that amount was in a state of chemical com- 

 bination not calculated for easy decomposition, consequently the 

 debris of such rocks aflForded but little lime in a soluble state. 



When, however, the subsequent and more basic class of rocks 

 made their appearance as traps, porphyries, diorites, some syenites, 

 &c., we find a supply of soluble lime provided for the develop- 

 ment of life and limestones. These rocks not only contained 

 much more lime than the granites, but were themselves much 

 2C2 



