Composition, and Analogies of Rocks, 39 



from the rounded and foreign fragments of discordant rocks 

 which it often contains. At the same time, there is no reason 

 to deny that it may have been exposed to the action of heat, as 

 it is : still capable of undergoing that without suffering any 

 change. That it was consolidated by heat we cannot prove ; 

 and are scarcely in a condition to deny, that it may not have 

 been partly indebted for its constitution to that cause. 



If shale could be indurated from water alone, there would ht 

 no reason to deny that the same cause may have operated in the 

 primary argillaceous schists ; while, that they have been depo- 

 sited from water, is proved by the fragments and the shells 

 which they so often contain. Here again, however, we are in 

 the same condition as with regard to quartz rock ; unable to 

 prove that it may not have experienced in some degree the 

 action of heat ; as we know, from observations on the siliceous 

 schists, that shells are not necessarily obliterated in these cir- 

 cumstances. But that action, if it existed, cannot have been 

 very great ; as we are certain, both from experiments and obser- 

 vation, that it is either fused or indurated by this cause. The 

 very existence of siliceous schist in the vicinity of trap and 

 granite, produced by the action of these rocks on shale and slatCf 

 not only prove this fact, but shew the very limits where the ac- 

 tion of heat ceases. 



Thus, two important members of the primary strata are, pro- 

 bably, indurated from water alone. With respect to limestone, 

 it is now known, both by direct experiment and by observation on 

 the effect produced by trap veins on chalk, that it may be crystal- 

 lized from fusion, provided the escape of the carbonic acid is re- 

 strained. It has been shewn, that it is equally consolidated from 

 water; and, on examining this limestone in its various associa- 

 tions, its origin must probably, in some instances, be referred to 

 one of these causes — in others, to the other. It is likely, for ex- 

 ample, that all the limestones associated with clay slate are de- 

 rived from watery deposition and crystallization ; and it is pro- 

 bable that those associated with gneiss have received their present 

 condition from heat. This opinion is justified by many circum- 



