58 Branner and Brackett — Peridotite of Arkansas. 



The presence of perofskite here is interesting as being the 

 third instance of its occurrence as a constituent of any Ameri- 

 can rock, the first instance being that reported by Dr. Wil- 

 liams in the Syracuse serpentine, the second that by Mr. J. S. 

 Diller, in the peridotite from Elliott County, Keutucky. It is 

 also interesting as occurring in the same type of rock as will 

 be mentioned later. 



The black grains scattered in not inconsiderable quantity 

 through the ground mass, are believed to be magnetite. The 

 yellow base ''looks as though it had been a glass once and 

 some of it is still isotropic, though most of it polarizes."* A 

 considerable amount of it is still isotropic as was found in 

 other sections (ISTo. 36 and 42.) From its mineral composition 

 and structure, then, this rock belongs to the family of perido- 

 tites, and to the new type of picrite porphyry or " Kiinber- 

 lite" of H. Carvill Lewis, f 



The rock differs somewhat from either of the other occur- 

 rences. Unlike the Kentucky peridotite it contains no ensta- 

 tite, its pyroxenic constituent being augite. It contains no 

 ilmenite, and in only one section was any garnet found, a 

 single, small, pink piece, quite isotropic. The perofskite, 

 especially, occurs in great abundance in the Pike County rock 

 and here is undoubtedly original, while in the Kentucky rock 

 it is believed to be secondary, arising from the decomposition 

 of the ilmenite, and the quantity is comparatively small. 

 Finally the Kentucky peridotite contains much more fresh 

 olivine than that from Pike County, and pyrope which is 

 abundant in the former is rare in the latter. The Syracuse 

 serpentine or peridotite, on the other hand, is much less fresh 

 than the Pike County rock, and while it contains augite in the 

 groundmass, the augites are much less abundant, as are also the' 

 perofskites. This rock is in some respects a new type. There 

 is total absence of a rhombic pyroxene, which occurs as such 

 in Mr. Diller's rock, and is probably represented by decompo- 

 sition products in the Syracuse serpentine. 



The brown rock, of which there is an exposure not far from 

 the picrite porphyry just described, shows in thin sections a 

 similar porphyritic structure. But here all the olivines are 

 changed to serpentine, carbonates and hydroxide of iron. The 

 outlines of the olivine and the structure of the rock are gener- 

 ally well preserved, although no fresh olivine remains. A 

 great many patches of mica, partially grown through with 

 colorless little augite crystals are present, and perofskite is 

 abundant. The most striking characteristic of the rock is the 

 almost total absence of augite in the groundmass. This 



* Dr. Williams on No. 35. 



f Rosenbusch, Mik.-Phys., vol. ii, p. 519. 



