VOLCANTC PIPES 359 



now record the effects of the eruptive breccia at different 

 stages in its cooling. 



As bearing upon the question of the origin of kimber- 

 lite, we must note certain basic and ultra-basic in- 

 clusions which are fragments of holocrystalline, and 

 sometimes granulitic, rocks, derived probably from great 

 depths in the crust of the earth. These rocks consist of 

 various combinations of the minerals olivine, enstatite, 

 brown mica, garnet, and more rarely ilmenite, felspar 

 and cyanite. They can conveniently be termed peri- 

 dotites, although certain combinations have been given 

 special names, e.g., eclogite, typically composed of 

 chrome-diopside and garnet ; saxonite, of olivine and 

 enstatite ; and Iherzolite, of olivine, diopside, enstatite 

 and garnet. 



By some geologists these inclusions have been re- 

 garded as " concretions " or " segregations " which have 

 formed in the magma of kimberlite during or prior to 

 its solidification, but there are numerous weighty objec- 

 tions to this view. Thus they are sharply defined from 

 the enclosing blue-ground and their boundaries are not 

 formed by the faces of their constituent minerals. 

 Again, they are holocrystalline, and are sometimes 

 foliated and show the effects of dynamo-nietamorphism. 

 Further, the eclogites, for example, can be shown to 

 belong to a peculiar group of rocks varying extremely 

 in composition and known as granulites, 1 and having 

 nothing in common with kimberlite. 



The minerals and mineral fragments may be divided 

 into three distinct groups according to their source,' 2 

 1 G. C., xiii. 2 G. C., xi., p. 151. 



