J. 8. Diller — Mica-peridotite from Kentucky . 287 



the dark green spots are serpentine and represent original oli- 

 vine. A thin section of the rock examined under the micro- 

 scope confirms the conjecture. 



The rock is composed essentially of biotite, serpentine, and 

 perofskite with a smaller proportion of apatite, muscovite, 

 magnetite, chlorite, calcite and other secondary products which 

 cannot be definitely determined. It is possible that originally 

 there may have been some pyroxene present. The biotite is 

 somewhat in excess of the serpentine and together they form 

 nearly 75 per cent of the whole rock, small crystals of perofskite 

 are abundant but their total mass is less than that of the brown- 

 ish-gray clouded material supposed to be of secondary origin. 

 Opaque black grains of magnetite are rather common and a 

 few clear crystals of apatite as well as scales of chlorite and 

 muscovite and small veins of fibrous calcite are present. Judg- 

 ing from the chemical analysis there is probably considerable 

 chromite present also. 



If we consider only the relation of the large scales of biotite 

 to one another as determining the structure of the rock it is 

 granitoid for the biotite is allotriomorphic. This mineral is 

 yellowish brown to almost colorless. It occurs in round or 

 oblong patches averaging about 4 mm in diameter and forms the 

 groundwork of the rock. Within these irregular scales are 

 scattered the crystals and grains of serpentine in such a way as 

 to render them pcecilitic on a cleavage face. The biotite here 

 plays essentially the same role as the hornblende and augite in 

 the picrites. Between the larger scales, scattered here a few, 

 and there many, throughout the brownish-gray clouded matter 

 are smaller scales of biotite. In sections of the biotite perpen- 

 dicular to its basal cleavage, although its colors between crossed 

 nicols are brilliant its absorption is very feeble even less than 

 that of ordinary brown hornblende. Prof. Rosenbusch sug- 

 gests that the absorption is too small and the bisectrix too 

 oblique for normal biotite. The biotite is occasionally altered 

 to chlorite. 



The serpentine is distributed uniformly throughout the rock 

 in irregular rounded or angular grains averaging about l mm in 

 diameter. The characteristic sections with outlines like those 

 of olivine parallel to the base and macropinacoid are sufficiently 

 numerous and well denned to clearly indicate that the serpen- 

 tine originated from the alteration of olivine. The large 

 quantity of olivine originally present shows that the rock 

 belongs to the peridotites. 



The mineral regarded as perofskite is abundant and generally 

 well crystallized. The diamond-shaped, square and triangular 

 sections which are completely isotropic demonstrate that it is 

 isometric. Its color is honey-yellow and only moderately 



