J. C. Branner — Paleozoic Sediments in Arkansas. 229 



Aet. XXX. — Thickness of the Paleozoic Sediments in Arkan- 

 sas ; by John C. Branner. With a Map (Plate III). 



The general divisions of the rocks of Arkansas are shown 

 on the accompanying geological map. There are no Archaean 

 rocks in the state, and the bottom of the Paleozoic beds is, 

 therefore, not exposed. The syenites of the state, formerly 

 supposed to be Archaean, are eruptives, probably of Tertiary 

 age ; whether the "red granite" said to occur at the mouth of 

 Spavina Creek in the Indian Territory, near the northwest 

 corner of the state, "^ is Archaean, is not known at present. 



The total thickness of the oldest sediments cannot, there- 

 fore, be determined from direct observations within the state, 

 and the total given in the present paper is, for that region, less 

 than it should be. 



On the other hand, the method of giving maximum thick- 

 nesses and of obtaining the total thickness of any one series of 

 beds by adding together outcrops observed at many different 

 localities, is liable to give an exaggerated idea of the total 

 thickness of sediments, for in all probability no such total 

 exists at any one place. 



The lowest rocks — the Lower Silurian — are exposed in 

 Arkansas in two separate regions : the first in the region of 

 the Ouachita uplifts, running west from Little Rock past Mt. 

 Ida to the Indian Territory ; the second north of the Boston 

 Mountains along the Missouri line. 



In the former area the rocks are exposed by erosion in a 

 region of sharp folds ; in the latter they are horizontal or nearly 

 so, are faulted in a few places, and are exposed for the most 

 part by the trenching of the streams and the exposure of per- 

 pendicular bluffs, where the character and thickness of the beds 

 may readily be inspected. 



In the region of the Ouachita uplift no attempt has been 

 made to subdivide the Silurian beds, further than to locate the 

 novaculites. Yery few fossils have ever been found in this 

 area and no correlation of the rocks with those of other Silu- 

 rian area of the state is possible at present, further than that 

 made by Dr. R. R. G-urley, based upon a study of the grapto- 

 lites found there. He says that a Trenton horizon and a Cal- 

 ciferous horizon are represented. f Mr. Griswold, who has 

 studied this area more thoroughly than any one else, says that 

 the shales, limestones, and quartzose sandstones below the 

 novaculite have a thickness of about 1300 feet, while the 



* Second report of a geological reconnoissance of Arkansas, by D. D. Owen, 

 Philadelphia, 1860, pp. 17 and 408. 



f Ann. Rep. Geol. Surv. Ark. for 1890, vol. iii, p. 401. 



