﻿124 J. & Diller — Peridotite of Elliot Co., Kentucky. 



yellowish ones are not as readily determined. They have a 

 high index of refraction and are generally spherical, but occa- 

 sionally their peripheries are straight and sharply defined, indi- 

 cating crystallographic form. Such grains usually possess a 

 higher degree of diaphaneity, and the sections are either trian- 

 gular, square or diamond-shaped. The latter are most strongly 

 doubly refracting, and extinction takes place parallel to the 

 longest diagonal. No trace of cleavage could be discovered, 

 but the facts enumerated render it highly probable that the 

 mineral is octahedrite. 



The relation of the peridotite to the Carboniferous sandstones 

 and shales is of paramount importance in determining its age 

 and origin. Only two reasonable hypotheses suggest them- 

 selves to my mind : (1) the peridotite may be older than the 

 Carboniferous strata and formed on the floor of the sea a peak 

 about which the horizontal strata were deposited ; (2) the 

 peridotite may have been erupted through the Carboniferous 

 strata. 



If the first hypothesis be correct we should expect to find 

 the adjacent sandstone composed largely of detritus derived 

 from the peridotite and to exhibit no evidence of contact meta- 

 morphism. On the other hand, if the second trypothesis be 

 true there would not necessarily be a correspondence in the 

 composition of the neighboring rocks and under favorable cir- 

 cumstances the sedimentary deposits would be metamorphosed 

 near their contact with the eruptive. Chemical analyses 6 and 

 4 in the following table are of the adjacent sandstone and 

 peridotite. The dissimilarity of the two rocks, both in chem- 

 ical and in mineralogical composition, is so prominent as to at 

 once dispel the thought that they are genetically connected. 

 Although the exact contact of the two rocks was not exposed, 

 hardened shale was found near the peridotite under such cir- 

 cumstances that its induration is certainly attributable to the 

 influence of the eruptive mass. But this is not the strongest 

 evidence, for the peridotite itself includes many fragments of 

 shale which were picked up. on its way to the surface. The 

 contact metamorphism has resulted generally in the develop- 

 ment of a micaceous mineral and the production from the 

 shale of a rock such as has been designated spilosite. On the 

 other hand, "the peridotite itself has experienced endomorphic 

 influences which resulted in the development to a very limited 

 degree of a sphserolitic structure similar to that of the varioles 

 in variolite. 



The chemical analyses given in the following table were 

 made with great care by T. M. Chatard, in the U. S. Geological 

 Survey laboratory at Washington. The specific gravities were 

 determined by the writer with a pycnometer in the case of the 



