60 
Kirtley F. Mather 
localities where fossils might be obtained. In the preparation 
of the manuscript my wife has rendered valuable assistance. 
Especially am I under obligation to Dr. Stuart Weller for the 
kind assistance he has so freely given during every stage of the 
work, in the identification of species, in the comparison with 
type and other fossils in the Walker Museum collections, and 
in the preparation of descriptions and illustrations, as well as 
for access to his library and manuscript bibliographies. 
Description of the Morrow Formations 
The series of limestones, sandstones, and shales which is 
known as the Morrow Group occupies a considerable area along 
the northern and northwestern slopes of the Boston Mountains 
in northwest Arkansas and northeast Oklahoma. In the type 
region, near the town of Morrow, Washington County, Arkan- 
sas, the group is composed of two formations: the Hale forma- 
tion below and the Bloyd shale above. 
The Hale formation ranges in thickness from about 100 feet 
to nearly 200 feet and consists of a variable series of sandstones 
and shales with lenses of rather pure limestone. Usually the 
basal portion is sandy shale, with thin layers of ripple-marked 
sandstone, which grades upward into a more or less massive 
calcareous sandstone. The relative amounts of sand and lime 
are by no means constant nor are the beds persistent in char- 
acter but change within short horizontal distances. 
The Bloyd shale resting comformably upon the Hale forma- 
tion ranges in thickness from 100 to 200 feet. It consists almost 
entirely of black, thin-fissile, carbonaceous clay shale of uniform 
character. Near the base of the Bloyd and separated from the 
Hale formation by 5 to 10 feet of the black shale is the Brent- 
wood limestone lentil. The latter generally consists of two, or 
at some places of three or more, beds of gray fossiliferous lime- 
stone, each from three to ten feet thick, separated by beds of the 
black shale in which it occurs. The upper part of the limestone 
is hidden in most places, but its total thickness, including the 
intervening beds of shale, may be as great as 40 or 50 feet. In 
the upper part of the Bloyd shale, generally within 60 or 75 
feet of its top, occurs the Kessler limestone lentil. It is a com- 
pact, gray to chocolate-colored limestone, in places conglomeratic 
