538 



PRELIMINARY REMARKS ON THE OCCASIONAL, 

 ROCKS. 



IT was remarked in the third chapter, that 

 this division was adopted for the purpose of com- 

 prehending some rocks which occurred both in 

 the primary and secondary classes, together with 

 the stony substances that are found in veins, and 

 which may thus exist indiscriminately in both. 



In a geological arrangement, like the present, 

 it is requisite to state more particularly the rea- 

 sons for adopting this expedient ; a proceeding 

 the more necessary, as the views on which it is 

 founded, are in a great degree at variance with 

 the current opinions and usual statements of 

 geological writers. 



It is not very easy to discover, in those 

 writings, in what light jasper has been consi- 

 dered, but it has chiefly been treated of as a 

 mineral, not as a rock ; while many substances, 

 most obviously appertaining to it, have been 



