THE PEEIBOTITES 



91 



Structure. — The structure as displayed in the different varie- 

 ties is somewhat variable. In the dunite it is as a rule even 

 crystalline granular, none of the olivines showing perfect crystal 

 outlines. In the picrites the augite or hornblende often occurs in 

 the form of bx^oad plates 

 occupying the interstices 

 of the olivines and wholly 

 or partially enclosing 

 them, as in the hornblende 

 picrite of Stony Point, 

 New York. The saxonites 

 and Iherzolites often show 

 a marked porphyritic 

 structure produced by 

 the development of large 

 pyroxene crystals in the 

 fine and evenly granular 

 ground-mass of olivines. 

 (See Fig. 5, as drawn 



hv Dr Cr TT Wil 



k/j x^x. \x. XX. vv ii- pjq^ 5^ — Mierostructure of porphyritic Iherzo 



liams.) Ihe rocks belong ixte, partly altered into serpentine. 



to the class designated as 



hypidiomorphic granular by Professor Eosenbusch; that is, 



rocks composed only in part of minerals showing crystal faces 



peculiar to their species. 



Colors. — The prevailing colors are green, greenish gray, yel- 

 lowish green, dark green to black. 



Nomenclature and Classification. — Mineralogieally and geo- 

 logically it will be observed the peridotites bear a close resem- 

 blance to the olivine diabases and gabbros, from which they 

 differ only in the absence of feldspars. Indeed, Professor Judd 

 has shown that the gabbros and diabase both, in places, pass by 

 insensible gradations into peridotites through a gradual dimi- 

 nution in the amount of their feldspathic constituents. Dr. 

 Wadsworth would extend the term peridotite to include rocks 

 of the same composition, but of meteoric as well as terrestrial 

 origin, the condition of the included iron, whether metallic or 

 as an oxide, being considered by him as non-essential, since 

 native iron is also found occasionally in terrestrial rocks, as 

 the Greenland basalts and some diabases. 



