Origin of Limestone. 313 



nodules in the limestone, and might easily be mistaken for the sili- 

 ceous infusible mineral that has been described by many writers un- 

 der the name of hornstone. But it melts with ebullition, not 'only 

 before the compound, but also the common blowpipe, into a shining 

 black enamel. I suspect that epidote enters largely into its compo- 

 sition. (No. 507.) It is probably the allochroite, mentioned by Prof. 

 Webster in the Boston Journal of Philosophy, as occurring at 

 Stoneham. 



I am not aware that limestone has been found in other parts of the 

 world, entirely embraced in sienite ; which is the case at Stoneham 

 and Newbury, unless I am mistaken. At the time I examined these 

 localities, I was not aware of the importance of this fact, and might 

 have been deceived. But I saw no rock in the vicinity of the quar- 

 ries, but sienite : although, as I shall have occasion hereafter to ob- 

 serve, sometimes the sienite north of Boston possesses a limited slaty 

 structure, forming a kind of hornblende slate, or greenstone slate : 

 the unmelted remnants, perhaps, of the rocks out of which the sienite 

 was formed. That this rock had an igneous origin, seems to be at 

 this day the prevailing opinion of geologists. And admitting this, it 

 is easy to see why the beds of limestone, that have been described 

 above, are destitute of stratification. 



Origin of Limestone. 



Crystallized carbonate of lime, as we are taught by chemistry, 

 may be produced either by precipitation from aqueous solution, or 

 by the melting of uncrystallized masses of this substance under 

 strong pressure : and probably in both these ways are we to account 

 for the existence of primary limestone. Where this rock is in reg- 

 ular strata, and no unstratified rocks in the vicinity, it is reasonable 

 to refer its origin to aqueous solution and crystallization. But where 

 there is evidence of the agency of heat, long continued, in the irreg- 

 ular position and unstratified structure of the limestone, and the jux- 

 taposition of granite, trap, &c. ; it is not improbable that limestones 

 deposited from water or animals, such as chalk, marl, compact lime- 

 stone, and coral reefs, may have been melted and subsequently crys- 

 tallized. 



Thus far most geologists agree. But another point divides them ; 

 viz. whether all limestones have originated in organized substances? 

 40 



