OCCASIONAL ROCKS. 543 



tions, which it was necessary to point out ; nor 

 could the subject be rendered intelligible, with- 

 out bringing the whole into one division. As 

 they belong both to the primary and the second- 

 ary class, they have necessarily found their place 

 in this part of the arrangement. 



The veinstones have been introduced into 

 this subsidiary place, solely for the convenience 

 of the reader, and because, had they not been 

 found here, they could not have been noticed in 

 any other part of this work. The geologi- 

 cal plan adopted, presents a reason for describ- 

 ing mechanical compounds which might other- 

 wise have passed unnoticed. They are frequent 

 sources of doubt and difficulty to students ; par- 

 ticularly where it is difficult to trace their con- 

 nections : and if it was necessary to bestow some 

 attention on the local conglomerates, which are 

 so intimately connected with them in character, 

 it was also important, in a geological view, to 

 prevent the two cases from being confounded. 



