Branner and Brackett — Peridotite of Arkansas. 58 



line of nearly vertical Cretaceous cliffs, which are the attacked 

 northern edges of these beds ; the left or northern border is 

 formed by the Paleozoic highlands, while the bottom of the 

 valley is in lower Cretaceous beds covered by Post-tertiary 

 debris and by Quaternary and recent sediments. It is in this 

 plain that the exposure of peridotite occurs. The accompany- 

 ing section shows the relations of the intruded rocks to those 

 of sedimentary origin. 



Section through the Pike County Peridotite and the adjacent Formations. 



I. River silt. IV. Lower Cretaceous (" Trinity " of Hill). 



II. Yellow loam. V. Paleozoic (lower Carboniferous ?) 



III. Post-tertiary. X. Peridotite. 



(The relations shown in this section, with the exception of the 

 exact contact of the Paleozoic with the igneous rocks, may all be 

 seen, though not at any one exposure.) 



The contact between the Paleozoic and the Cretaceous is 

 exposed in Prairie creek about two miles northeast of Mur- 

 freesborough where the Cretaceous rock is a conglomerate with 

 calcareous cement. These parti-colored Cretaceous beds are 

 cut into and exposed in many places, and at low water almost 

 continuously, along Prairie creek from this point to the mouth 

 of the stream, while on the right bank of the Little Missouri 

 they rise in beautifully exposed cliffs to a height of nearly one 

 hundred feet above the river. 



Where Prairie creek enters the Little Missouri, a dyke of 

 peridotite not more than ten inches wide stands out for fifty 

 feet across the mouth of the former stream, and on the left 

 bank of the river this dyke is seen to penetrate the soft sand- 

 stones of the lower Cretaceous. Where the Cretaceous has 

 been cut away by Post-tertiary erosion and covered with the 

 water-worn debris, the dyke is also cut off even with the eroded 

 Cretaceous surface and covered with debris. At the line of 

 contact between the dyke and the Cretaceous sandstone, the 

 most careful microscopic examination does not reveal the 

 slightest trace of metamorphism. The original material injected 

 into this crevice is so thoroughly filled with the debris of the beds 

 through which it has passed — shales, sandstones and quartz 

 pebbles — that their included fragments form about two-thirds 



