﻿Clarke and Diller — Turquois from New Mexico. 217 



their genetic connection. Pyrite is scattered rather uniformly 

 throughout the rock in small cubical crystals easily seen in the 

 hand specimen. They are sometimes altered to limonite, but 

 in other cases they have been completely replaced by pseudo- 

 morphs of epidote. The specimen of rock which was subjected 

 to analysis probably contained an amount of pyrite rather 

 greater than the average. 



One of the most important constituents of the rock, because 

 of its very close association with the turquois, occurs in the 

 form of bright yellow grains. They are not distinctly pleo- 

 chroic, but have a high refractive index, with strong double 

 refraction, and give brilliant aggregate polarization. The parti- 

 cles are usually very small, and grouped together in irregular 

 compound grains so as to suggest nothing of crystallographic 

 form. In several cases, however, elongated simple grains show 

 inclined extinction ; which indicates that the mineral must be 

 either monoclinic or triclinic in crystallization. Judging not 

 only from the properties enumerated, but also from the per- 

 centage of lime in the rock, this mineral is in all probability 

 epidote. It is evidently connected genetically with the tur- 

 quois, for it is almost uniformly found upon the border of the 

 latter, and is most abundant in its neighborhood. 



Concerning the origin of the turquois-bearing rock, it may 

 be stated that Professors Newberry and Silliman, both of whom 

 studied it in the field, regard it as eruptive, and probably of 

 Tertiary age. The occurrence of this veritable orthoclase rock 

 in the West is of special interest from the facts disclosed by 

 recent investigations that in many of the rocks previously 

 described as trachytes the predominating feldspar is plagioclase. 



The very small size of the veins and their limited distribu- 

 tion show that the turquois is of local origin, and emphasizes 

 the idea that it has resulted from the alteration of some other 

 mineral. In addition to the evidence already cited to show 

 that the turquois has been derived from apatite, we have the 

 fact that epidote, a lime- bearing mineral, is present as a second- 

 ary product. The oxidation of pyrite may have had some- 

 thing to do with initiating the process of alteration, and the 

 alumina of the turquois was probably derived from decompos- 

 ing feldspar. The latter suggestion was made by Silliman, 

 who- also examined microscopic* sections of the rock, and 

 reported apatite as present. No apatite, however, could be 

 seen in the specimens examined by us. A search for it at the 

 locality would certainly seem to be desirable. 



U. S. Geological Survey, Washington, April, 1886. 



