﻿122 J. S. Diller — Peridotite of Elliot Co., Kentucky. 



tains those derived from sedimentation. It is in the latter 

 group that Kalkowsky places the peridotites. They nearly 

 always occur intimately associated with highly altered rocks in 

 regions of great disturbance, so that their relations cannot be 

 readily determined. In eastern Kentucky, however, they are 

 found with nearly horizontal unaltered sandstones and shales 

 of the Carboniferous series and present such a promising oppor- 

 tunity for completely demonstrating the origin of peridotite 

 that a more thorough examination has been undertaken. 



The peridotite is a compact dark greenish rock with a spe- 

 cific gravity of 2*781. In it are embedded numerous grains of 

 yellowish olivine uniformly distributed throughout the mass. 

 Rarely it is fine-granular and dense like many dark colored 

 basalts, but generally the grains of which it is composed are 

 medium-sized. Occasionally the olivine grains wholly disap- 

 pear and the deep green serpentine pervades the whole mass. 

 Besides olivine and serpentine, which together form nearly sev- 

 enty-five per cent of the rock, there are prominent grains of 

 pyrope and ilmenite with a few scales of biotite. 



The following table, showing the mineralogical composition 

 of the peridotite, is based directly upon estimates made under 

 the microscope of the areal distribution of the various minerals 

 in the freshest portion of the sections taken from the locality 

 where the peridotite is least altered. 



Primary minerals. Secondary minerals. 



Olivine 40 per cent. Serpentine 30*7 per cent. 



Pyrope 8 " Dolomite.."- -.14 " 



Biotite 1 " Magnetite.,_ 2 " 



Enstatite _. 1 " Octahedrite _ 1*1 " 



Ilmenite ___ 2*2 " 



Apatite trace. 



It is not claimed that this table represents with a high degree 

 of accuracy the composition of the rock, but it closely approxi- 

 mates the real proportions in the sections studied. The table 

 clearly indicates that originally at least eighty per cent of the 

 peridotite was olivine, and that ultimately it will be nearly all 

 serpentine, or perhaps in some places dolomite, with a small 

 proportion of magnetite, ilmenite, pyrope and octahedrite. 



The olivine generally occurs in the form of irregular grains 

 held in the network of serpentine and other secondary products, 

 but rarely, however, it is bounded by well defined crystallo- 

 graphic planes, a feature which is somewhat unusual for the 

 olivine in peridotites. The crystals are short prisms terminated 

 by brachydomes like those so commonly seen in basaltic lavas. 

 Enstatite plays so small a part among the minerals of this 

 rock that it cannot be considered an essential constituent. It 

 occurs in the form of irregular corroded grains with clouded 



