542 OCCASIONAL ROCKS. 



substances described in it, with their geological 

 history, it seemed expedient to adopt such a 

 course as would tend most to illustrate that main 

 end of the study of rocks. 



It was with some hesitation that gypsum was 

 admitted into this catalogue. Like salt, it might 

 have been, perhaps with equal propriety, re- 

 ferred to the list of independent minerals ; had 

 not the practice of geologists established for it a 

 place, for which the large masses in which it 

 occurs offer some justification. Like many other 

 doubtful points in this arrangement, it must wait 

 the event of future correction : but as it is found 

 both among the primary and secondary strata, 

 and without any differences of character, it seemed 

 more convenient to place it in this part of the 

 classification, than to repeat it in each. 



Of the conglomerate rocks, although some 

 have already come under review, there remain 

 others, chiefly of a local nature, which no oppor- 

 tunity was afforded of introducing in a detailed 

 manner. The general and the local rocks of 

 this construction, are moreover distinguished by 

 important differences in their geological rela- 



