« 





• 







m 



















* 





Cii. VIII.] 



TEXTURE OF OLDER AND NEWER ROCKS. 



141 



formation 



But, on the other hand, we observe in certain 

 now in progress, particularly in coral reefs, and in deposits 

 from the waters of mineral springs, both calcareous and 

 siliceous, that the texture of rocks may sometimes be stony 

 from the first. This circumstance may account for exceptions 

 to the general rule, not unfrequently met with, where solid 

 strata are superimposed on others of a plastic and incoherent 

 nature, as in the neighbourhood of Paris, where the tertiary 



com 



aw-*-— — • - j \-_x — 



grit, are more stony than the subjacent chalk. 



It will readily be understood, that the various solidifying 

 causes, including those above enumerated, together with the 



incumbent 



and the influence of subterra- 



them 



their full power. 



If in the course of ages they modify the 



rive t 



aspect and internal structure of stratified deposits, they will 



ise to a general distinctness of character in the older as 

 contrasted with the newer formations. 



Transition texture. — In the original classification of Werner, 

 the highly crystalline rocks, such as granite and gneiss, 

 which contain no organic remains, were called primary, and 

 the fossiliferous strata secondary, while to another class of an 

 age intermediate between the primary and secondary he gave 

 the name of transition. They were termed transition because 

 they partook in some degree in their mineral composition of 

 the nature of the most crystalline rocks, such as gneiss and 

 mica-schist, while they resembled 



containing occasionally organic 



remains, 



terous series m 

 and exhibiting 



evident signs of a mechanical origin. It was at first imagined, 



intermediate 



deposited subsequently to the series called primary, and 



more earthy and fossiliferous formations 



But 



organic remains 



when the relative position and 



transition rocks were better understood, it was perceived that 



they did not all belong to one period. On the contrary, the 



i 



in the Alps, which 



same mineral characters were round in s 



ages, and some formations occurring 



several of the ablest scholars of Werner had determined to be 



transition, were ultimately ascertained, by means of their 



