46 DESCRIPTION OF THE PRIMARY [Ch. IV. 



diallage differs from hornblende in containing a large pro- 

 portion of this earih. 



The constituents of euphotide, in some places, gradually 

 become so small, that the rock is at length perfectly homo- 

 geneous ; and where this is interlaminated with small scales of 

 shining diallage, the rock acquires a schistose structure. This 

 schistose euphotide has been commonly called hornblende 

 slate ; but it is so different from all the varieties of the latter, 

 and has such a characteristic appearance, that it ought to be 

 distinguished. It appears, sometimes, to be imbedded both 

 in the euphotide and in the serpentine ; and the latter rock, 

 even in some instances, seems to be subordinate to the schist. 

 The fact is, that the schist often forms the connecting link 

 by which euphotide and serpentine graduate into each other ; 

 and though these rocks alternate on a given line, like the 

 massive and schistose greenstones, yet they do not preserve a 

 continuous course for any considerable distance ; but mutually 

 abut against, or are dovetailed into, each other ; sometimes 

 the one, and sometimes the other prevailing. 



The serpentine of Cornwall is proved to be a compound of 

 diallage and felspar ; or perhaps, rather, of compact felspar, 

 by its frequent transitions into euphotide. This rock ex- 

 hibits a great many varieties ; some of which are hard, whilst 

 others are so soft as to yield to the nail. This difference ap- 

 pears to depend on the felspar base, which undergoes several 

 modifications, between a crystalline, compact, and granular 

 state, as seen in the precious, common, steatitic, and ollareous 

 serpentines ; in the same manner as the rocks of the porphy- 

 ritic group assume various aspects, according to the composi- 

 tion of the compact felspar base ; with this difference, however, 

 that in these, the proportion of the silica modifies the com- 

 pound, whereas, in the serpentine, the changes are attributable 

 to the relative quantity of magnesia. The accessary mineral, 

 diallage, also, imparts characters to the serpentine, according 

 as it is intimately combined with the base, or is disposed in 

 distinct forms. And, lastly, both of the component minerals, 



