This ill-defined group includes those rocks which are extremely poor in silica, and which are, as a rule, may be looked upon as basic rocks with an unusually large proportion of ferro-magnesian silicates. The rocks do not admit of such a satisfactory grouping as those previously considered.

            Many names have been given to individual rocks which differ very slightly from one another, but they may conveniently be placed into groups, the group name indicating the nature of the rock satisfactorily. The tendency to crystallize is still more strongly marked than in the basic rocks.

A. Plutonic and Hypabyssal

Pyroxenites consists largely pyroxene.

Horblendites consist largely of hornblende.

Eclogites consists largely of green pyroxene, green amphibole, and red garnets.

Picrites consists of olivine, augite, and sometimes a little feldspar. Some varieties contain hornblende in addition to, or instead of, the augite.

Peridotes consists largely of olivine.

Serpentines consists largely of serpentine which may have been produced by the alteration of olivine, pyroxene, or amphibole.

 

 

B. Volcanic

 

Augitite consists of augite and magnetite.

Limburgite consists of augite, magnetite, and olivine.