52 Bra/nner and Brackett — Peridotite of Arkansas. 



inches wide and about fifty feet lone:, while a deep gully on 

 the north side of the outburst (Poor House branch on the 

 map), shows no exposures. East of the exposure, at the house 

 of Mr. McBrayer (shown upon the accompanying map), a well 

 recently dug to a depth of 162 feet penetrated only the clay, 

 cobble stones and soft calcareous beds, such as characterize the 

 Post-tertiary and lower Cretaceous in this region. 



The following is the section of Mr. McBrayer's well: 



1 ' Clays Quaternary and recent. 



10' Cobbles and pebbles .-Post-tertiary. 

 142 ' Soft arenaceous clays ) 



and calcareous beds > Lower Cretaceous ("Trinity" of 

 variously colored. ) Hill). 



These facts appear to indicate that the outburst was confined 

 to this very circumscribed area, there being no eastward exten- 

 sion of it at the depth reached in McBrayer's well — 162 feet 

 below the level of his house — and but one westward exposure 

 uncovered at the mouth of Prarie creek. 



Relations of the igneous to the sedimentary rock. — Besides 

 this peridotite, the rocks exposed in this part of Arkansas are 

 of Paleozoic, Lower Cretaceous (" Trinity " of Hill ), Post-ter- 

 tiary and Quaternary ages. The Paleozoic rocks form the high 

 lands of the hilly and mountainous region of the state lying 

 north of the Neozoic exposures. They are made up of alter- 

 nations of sandstones and shales, and are highly flexed, the axes 

 of the folds varying but little from due east and west. Just 

 north of Murfreesboro, and four miles from the exposure of 

 peridotite, these sandstones and shales have a high south dip, 

 at many places standing almost or quite vertical. These south 

 dips continue for many miles to the north, a section measured 

 Across the beds farther east showing an aggregate vertical thick- 

 ness of strata of at least four miles. Against and upon the 

 eroded, upturned edges of these Carboniferous rocks the lower 

 Cretaceous beds have been deposited. The rocks of the Cre- 

 taceous are soft sandstones, shales, lignites, clays, etc., all beau- 

 tifully variegated, the predominating colors being straw, lead, 

 pink, and terra cotta, and the beds exhibit a low and almost 

 imperceptible dip to the east and southeast.* 



The Little Missouri River and its predecessor, flowing along 

 the original inland margin of the Cretaceous, have here cut out 

 a valley five miles wide, its right and southern wall being a 



* Through the kindness of the Director of the U. S. Geological Survey, Prof. 

 R. T. Hill spent the past year in studying the Mesozoic geology of Arkansas. 

 His report is already completed and forms Vol. II of the annual report of the 

 Geological Survey of Arkansas for 1888. In this report, Prof. Hill shows that 

 the Mesozoic rocks in the vicinity of this exposure belong to what he calls the 

 Trinity, which he thinks is equivalent of the Wealden of Europe. 



