92 BOOKS FOBMED THBOUGH IGNEOUS AGENCIES 



In classifying the peridotites the varietal distinctions are 

 based upon the prevailing accessory mineral. We thus have : 



Dunite, consisting essentially of olivine only. 



Saxonite, consisting essentially of olivine and enstatite. 



Picrite, consisting essentially of olivine and augite. 



Hornblende picrite, consisting essentially of olivine and hornblende. 



Wehrlite (or eulysite), consisting essentially of olivine and diallage. 



Lherzolite, consisting essentially of olivine, enstatite and augite. 



The name Dunite was first used by Hoehstetter and applied 

 to the olivine rock of Mount Dun, New Zealand. Saxonite 

 was given by Wadsworth, rocks of this type being prevalent in 

 Saxony. The same rock has since been named Harzhurgite by 

 Rosenbusch. The name Lherzolite is from Lake Lher^: in the 

 Pyrenees. 



The peridotites are, as a rule, highly altered rocks, the older 

 forms showing a more or less complete transformation of their 

 original constituents into a variety of secondary minerals. The 

 most common result of this alteration is the rock serpentine, the 

 transformation taking place through the hydration of the olivine 

 and the liberation of free iron oxides and chalcedony. ( See Fig. 5. ) 

 The chemistry of the process has been already discussed under 

 the head of olivine, p. 23. The prevailing color is some shade of 

 green, though not infrequently brown, yellow, red, or nearly 

 black. 



(2) THE PICBITE POBPHYBITES 



Under this head is placed a small group of rocks so far as 

 now known very limited in their distribution, which are regarded 

 as the effusive forms of the plutonic picrites, as bearing the same 

 relation to these rocks as do the melaphyrs to the olivine diabases. 

 The essential constituents are therefore olivine and augite with 

 accessory apatite, iron ores, and other minerals mentioned as 

 occurring in the true picrites. Structurally they differ from 

 these rocks in presenting an amorphous base rather than being 

 crystalline throughout. The group is quite limited in the United 

 States. Elliott County, Kentucky ; Pike County, Arkansas ; and 

 Syracuse, Onondaga County, New York, are well-known occur- 

 rences. 



