75 



as they will, like the former, be necessarily 

 examined in the chapter on the structure of 

 rocks. 



The two classes of primary and secondary, 

 are followed by a division termed Occasional 

 rocks. This indispensible irregularity in the 

 present arrangement, has been adopted for the 

 purpose of comprising some rocks which occur 

 indiscriminately in both classes, together with the 

 substances of a stony nature which are found in 

 veins, and are therefore of posterior origin to the 

 strata, whether primary or secondary ; in both of 

 which they occur. 



To the whole are subjoined two appendices. 

 The first contains the volcanic rocks, which, if 

 they are not in every instance decidedly distin- 

 guishable in character from the rocks of the trap 

 family, are sufficiently distinct in their origin to 

 deserve a separate consideration. 



The substances contained in the second ap- 

 pendix might perhaps have been omitted in an 

 arrangement professing to treat of rocks ; but as 



