274 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



interstratified with the black shale in the upper part of the formation. At various positions 

 in the section, especially in the more cherty beds, are small, rqunded, marble-like concretions 

 of a calcareous nature. In places there are large segregations of a similar character, concen- 

 trically banded, which occur intersecting several layers of cherty shale. 



Fossils are very rare in the Woodford chert, and those found are not well preserved. In 

 the lower part of the formation in the western part of the region a small Lingula of the type 

 L. spatulata Hall and a few conodonts have been observed. In the shales of the upper part the 

 only recognizable fossils seen are two concentrically plicated species of Productella, one of 

 wliich seems referable to P. concentrica Hall. 



This formation is believed to be of the age of the Chattanooga formation of Tennessee, 

 the Oliio shale of Ohio, and the Portage and Chemung of New York. At the top it doubtless 

 includes strata corresponding in age with the Noel shale of northern Arkansas and the basal 

 shale of the Tullahoma formation of Tennessee, which are believed to be of Kinderhook age, or, 

 according to the present classification, earhest Carboniferous. 



At present it is not possible to determine the exact parting between the Devonian and 

 Carboniferous, since there is no clear stratigrapliic distinction in the shales, and fossils are rare. 



I 15. ARKANSAS. 



The Chattanooga shale (Devonian) is represented in northern Arkansas ^' *'° 

 together M^ith a local basal member known as the Sylamore sandstone. The forma- 

 tion consists of a persistent black shale, with locally a dark phosphatic sandstone 

 or conglomerate (Sylamore member) at the base, and represents a late stage of 

 advance of the sea upon the Ozark land. In places it reaches a maximum thickness 

 of 150 feet, but the areas are too small to be shown on the map. 



I 16, J 17. THE APPALACHIAN VALLEY (ALABAMA, GEOE.GIA, TENNESSEE, THE VIRGINIAS, 



AND MARYLAND). 



Along the trend of the Appalachian Valley and Mountains the Devonian 

 formations exhibit a very marked change. In Maryland their sequence is still 

 complete from the Helderberg limestone up and their thickness in Allegany County 

 is 7,875 to 10,200 feet; but they thin out toward the south, the lower formations 

 appear only locally or not at all, and in Georgia and Alabama the Upper Devonian 

 (Chattanooga shale) overlaps unconformably upon older strata and is reduced to 

 a thickness of. 10 or 15 feet only. 



The Maryland section is typical for adjacent areas of Pennsylvania and Vir- 

 ginia. Prosser ''**'°' in 1901 summed up the results of investigations carried on by 

 the Maryland Survey. He describes the Helderberg limestone (upper part of the 

 Lewistown limestone), as thin-bedded bluish-gray ringing liniestone, more or less 

 shaly, 400 feet thick, followed by massive darker-blue limestone to a total thickness 

 of 750 to 900 feet. The strata are fossiliferous and according to Prosser comprise 

 representatives of the Manlius, Coeymans, New Scotland, and Becraft limestones of 

 the New York section. The Cayugan strata (Manlius of Prosser) are now separated 

 from the Helderberg in Maryland. 



The Oriskany sandstone Prosser describes as bluish-black cherty limestone, 

 75 to 100 feet in thickness, above which comes a white calcareous sandstone 



