56 Brainier and Brackett — Peridot it<: <f Arkansas. 



ever, by Tertiary beds. The Magnet Cove crystallines are also 

 in Paleozoic rocks, and are overlain here and there by Post 

 tertiary debris. 



Part II. A Microscopic Study of the Peridotite of Pike 

 County, Arkansas, by Richard N. Brackett. 



The specimens of eruptive rock from the middle hill shown 

 in the map, consists in the main, of a dark colored somewhat 

 green, heavy rock having a porphyritic structure, and sj)ecific 

 gravity of 2*728 to 2*651. Examined microscopically it is seen 

 to be made up of black grains, some slightly yellow and having 

 glistening surfaces, imbedded in a dark green to brownish-green 

 groundmass. The material from the base of the northeastern 

 hill, is a brown,, much decomposed rock, with a more distinctly 

 porphyritic structure due to the decomposition of the black to 

 yellow grains, and of the groundmass to a decided brown, 

 against which the yellow grains stand out sharply. The specific 

 gravity of this rock is 2 - 317. Through it extends a vein of 

 white barite about four inches in thickness. 



In contact with the barite vein are veins of serpentine formed 

 by the decomposition of the rock. In immediate contact with 

 the barite the serpentine vein is white, but shades through a 

 light green into the brown rock. 



A microscopic study of thin sections prepared from speci- 

 mens from the first exposure mentioned, reveals a rock of true 

 porphyritic structure, consisting of crystals and grains of more 

 or less decomposed, colorless olivine and some irregular patches 

 of a yellow to brownish-yellow mica imbedded in a quite uni- 

 form, fine-grained groundmass made up of colorless little lath- 

 shaped crystals, yellow grains, black grains and a yellowish base 

 (Nos. 34, 35 and 36).* 



The olivine crystals and grains are decomposed in the usual 

 well-known way, being cracked and changed to serpentine 

 along the cracks. Few or none of the olivines are entirely un- 

 changed, though there are many fresh cores and almost entire 

 grains and crystals remaining. (No, 35). Where no olivine is 

 left the outlines of the former olivine crystals are often well 

 preserved. In such cases the olivines are entirely changed to 

 serpentine, of both yellow and light green color and to carbon- 

 ates, and hydroxide of iron, to which last the reddish stain of 

 many is due (Nos. 34 and 36). Many of the decomposed oliv- 

 ines contain also " trichites," slender, black, hairlike bodies 

 which occur singly and in bunches. These " trichites " are 

 probably magnetite. 



*Numbers in parenthesis refer to numbers of specimens in the collections of 

 the Geological Survey of Arkansas. 



