﻿J. S. Diller — Peridoiite of Elliot Co., Kentucky. 123 



borders and is distributed throughout the mass with approxi- 

 mate uniformity. 



Next to the olivine, pyrope is the most important constituent 

 of the rock. It occurs in the form of spherical or elliptical 

 grains varying from one to more than a dozen millimeters in 

 diameter. They are found abundantly along the line of the 

 dike in the soil resulting from its disintegration. The small 

 clear deep red grains have a specific gravity of 3*673 and are 

 locally regarded as rubies of problematical value. The most 

 interesting feature of the pyrope is prominent under the micro- 

 scope, where it is seen to be surrounded by a border of radial 

 fibres exactly analogous to those described by Schrauf* as 

 Kelyphite and later critically examined by A. von Lasaulx.f 

 In this case the border is composed of several essentially 

 different substances which are always present although varying 

 much in proportions. First of these may be mentioned a dark 

 powder of magnetite which is frequently so abundant upon the 

 outer edge of the border as to render it opaque. The inner 

 substance of the ring is of a grayish or reddish brown color 

 and generally fibrous in structure, perpendicular to the periph- 

 ery of the garnet. The fibres have occasionally very strong 

 absorption in the direction of the longer axis and have nearly 

 or quite parallel extinction, indicating that the mineral is biotite. 

 This conclusion is completely demonstrated by a border in part 

 of which the uniaxial, negative, strongly clichroic folia of biotite 

 may be clearly discerned. The biotite extends far into the fis- 

 sures of the garnet and evidently results from its alteration. 

 Associated with it are small triangular and quadratic sections 

 of a yellowish brown isotropic mineral, which in all probability 

 is picotite. Lasaulx clearly demonstrated the presence of 

 pyroxene and amphibole, in the so-called Kelyphite. BeckeJ 

 and Kalkowsky§ pointed out the picotite but so far as I am 

 aware the presence of biotite in this connection is here noted 

 for the first time. 



The ilmenite is readily distinguished from the magnetite even 

 under the microscope in reflected light by the brilliant coaly 

 luster of portions of its pitted surface. It occurs in large 

 grains and not in the form of spongy particles like the magne- 

 tite. Under the microscope the ilmenite is frequently seen 

 surrounded, penetrated, and even completely replaced, by a 

 mixed group of yellowish and black grains, resulting from its 

 alteration. The black opaque grains are magnetite, but the 



* " Ueber die TTmrindungen von G-ranat." Sitzungsberichte der Niederrhein* 

 Gessell. zu Bonn. 1882, July 3. 



f " Beitrage zur Kenntniss des Associations Kreises der Magnesiansilicate," 

 Zeitschriffc fur Krystallographie, 1882, vi, 321-388. 



\ Tschermak's Mineralog. und Petrographische Mittheilungen, iy, pp. 189, 285. 



§ Elemente der Lithologie, p. 238. ■ 



