208 PRIMARY LIMESTONE. 



inary or secondary nature of a calcareous rock; from 1 a 

 detached specimen. 



It lias often been agitated whether all limestones 

 had or had not originated in organized substances , 

 That question has already been considered in the 

 twelfth chapter ; and it is probable that many of the 

 primary limestones have once contained organic re- 

 mains, like the secondary, even where these can no 

 longer be detected. In Airdriamurchan, facts similar 

 to those in Sky and Mann occur under some variation ; 

 conchiferous strata being converted into crystalline 

 limestone or chert, according to their previous charac- 

 ters, with the disappearance of their organic bodies, at 

 the places where trap overlies or intersects them, and 

 no where else. In these cases, therefore, the influence 

 of trap, under different modes, has not only caused a 

 secondary limestone to assume the crystalline texture, 

 but obliterated the forms of the beds, while the organic 

 bodies have disappeared. The instance from the Isle 

 of Mann, especially, seems to prove that they had been 

 combined with the mass of the rock by fusion ; and, 

 that this is a process of fusion, is proved by the analo- 

 gous effects produced on chalk by the passage of trap 

 veins : while, though no large mass of trap is found in the 

 Isle of Mann, to account for the appearances, the nu- 

 merous veins offer sufficient evidence of former bodies 

 of that rock whichjiave been destroyed in the progress 

 of time. The case of Airduamurchan is peculiarly 

 explicit. But it is here almost superfluous to repeat, 

 that the influence of granite, of gneiss, or of other 

 rocks which have been under the action of heat, may 

 have equally caused the obliteration of the organic 

 remains where they chanced to exist ; producing also 

 the crystalline texture common in these limestones. 

 And that this actually docs occur, is proved by the 



