34^ TJie A7nerica7i Geologist. .juuo,i898 
coal measures" from being considered systematically. The 
first important reference was in regard to the thickness, the 
estimate being placed at 1,200 feet.* The latest suggestion 
in this connection is by Norton,t who places the total thickness 
of the "lower" and "upper coal measures" combined, at 1,060 
feet. In all of these references the plane of separation between 
the "upper" and "middle," or the "upper" and "lower coal 
measures" — the "middle" not being recognized- — is understood 
to be that selected by White. It is not distinguishable at any 
other locality than the one noted by him, and is about 75 feet 
below the horizon now adopted for the division plane between 
the Des Moines and Missourian series. 
As the result of investigations carried on in Missouri± the 
base of the Bethany limestone was found to be an horizon 
that afiforded the greatest contrasts of all essential characters. 
It properly formed the division between the two principal 
series of the region. The Bethany limestone was later cor- 
related with the Winterset limestone of Iowa. Since that time 
Bain§ has traced the outcrops of the formation all the way 
from Guthrie county, in the central part of the state, south- 
W'ard to the typical locality in Missouri. 
The fact that the "upper coal measures" of Iowa, which 
are now incorporated in great part in the Missourian series, 
were never subdivided is due to a number of circumstances. 
The vertical extent and importance of the formation was not 
recognized; heavy accumulations of drift totally obscured the 
rocks; the region occupied by the- strata is mainly a water- 
shed, and hence no large streams exist to cut down to bed- 
rock. In order to decipher the Iowa district it was necessary 
to approach it from some other direction than had been hither- 
to attempted, — from some locality in which the entire sequence 
had been clearly made out. This key was furnished by the 
work done in Missouri, and along the Missouri river. There 
the full succession of strata had been recently determined, and 
some of the formations traced into Iowa. 
The subdivisions of the Missourian series, as developed in 
*Iowa Geol. Surv., vol. I, p. 15, 1893. 
tibid., vol. VI, p. Z2>Z, 1897. 
JMissouri Geol. Sur., vol. IV, p. 82, 1894. 
§Iowa Geol. Sur., vol. VIII, p. 25, 1898. 
