77 



CHAP. IV. 



GENERAL CATALOGUE OF THE FAMILIES OF 

 ROCKS CONTAINED IN THE PRESENT AR- 

 RANGEMENT. 



IN the following catalogue, the several fami- 

 lies of rocks are arranged according to the divi- 

 sions which have been discussed in the preced- 

 ing chapters. The reasons for these divisions 

 have already been examined : those on which the 

 nearly arbitrary succession of the families has 

 been founded, will be treated of in the subse- 

 quent chapter. Their very unequal importance 

 and extent is very striking in the case of talcose 

 and of actinolite schist, compared with gneiss or 

 with the overlying rocks. But the increase of 

 such families, or genera, is so much more desir- 

 able than their diminution, that it seems expe- 

 dient to adopt any reasonable excuse for multi- 

 plying them, wherever that can be borne out by 

 their mineral characters and their geological con- 



