Branner and Brackett — Peridotite of Arkansas. 51 



close of the Cretaceous, sank the greater part of Arkansas as 

 well as the large Tertiary-covered portions of the neighboring 

 states beneath the ocean. It is important, also, from a petro- 

 graphic standpoint as being the third reported occurrence of 

 picrite-porphyry in the United States. 



"With the accompanying map and section before the reader, 

 it will not be necessary to give a detailed description of the 

 locality. It was first reported by Dr. Owen in his " Second 

 Geological Report," p. 82, but was not studied by him in 

 detail, and the rock is simply spoken of by him as a " porphy- 

 ritic greenstone" and a "trachytic rock." Since Dr. Owen's 

 time no one seems to have made any observations upon it. 

 Some of the geological maps that have, from time to time, 

 been published of the United States, have represented in this 

 place a large Archaean area. The rock presents no great 

 variety in lithologic characters, and the specimens examined 

 microscopically by Dr. Brackett, and described by him in the 

 second part of this paper, fairly represent them, except that in 

 many places through the general mass it contains a good many 

 angular and sub-angular inclusions of crystalline rock, which 

 are especially noticeable wherever the rock is deeply decom- 

 ]30sed, and that one small dyke coming up through the Meso- 

 zoic beds contains a vast quantity of fragments of Paleozoic 

 sandstone and shale and of soft sandstone and quartz pebbles 

 from the Mesozoic. Only in the three hills shown upon the 

 map, and in the one very small dyke is the rock found solid, 

 disintegration having gone so far in the lower grounds that it 

 there occurs only in the form of a soft earthy wacke, which 

 washes very readily into deep gullies. This earth, where 

 freshest and unmixed with organic matter, presents many 

 beautiful shades of green, brown, red, and gray colors. At 

 one point a dyke is uncovered in one of these gullies. This 

 dyke is about six feet wide, runs east-west, and in place of 

 the olive-green color so characteristic of the general mass, it is 

 of a beautiful bright blue color. It is so deeply decomposed 

 that no solid specimens could be had from it. On the summit 

 and sides of the central hill the rock mass is broken into large 

 blocks, which, by concentric disintegration and exfoliation, are 

 left in the form of bowlders of various sizes. 



If the overlying Post-tertiary and Quaternary debris could 

 be removed in the immediate vicinity of this exposure, it is 

 probable that the area of igneous rocks, as shown upon the 

 accompanying map, would be somewhat enlarged, at least by 

 disclosing dykes radiating from the central mass. There is no 

 reason for supposing, however, that the Post-tertiary obscures 

 any great area of peridotite. There are no exposures of it in 

 Prairie creek, except a single small dyke not more than ten 



