THE MIDDLE COAL MEASURES 579 
least constant in character, maintain themselves over areas of 
many miles. All of the beds, whether they be limestone made 
up of beach-rolled fragments, shale suggesting the infra-littoral 
zone of deposition, or the pure heavy limestone probably mark- 
ing open sea deposition, are of such uniformity as to force the 
conclusion that only under conditions of widespread stability 
could they have been formed. In Kansas and the southern 
portion of the field there was an important recurrence of shore 
conditions later in Missourian time. In the portion of the field 
immediately under discussion, however, this later period of 
shore conditions is much less important and the uniformity 
which marks the opening of the Missourian, seems to have 
persisted throughout the period. 
The change from the condition obtaining in the early Des 
Moines to those present when the Missourian began was a 
gradual one. During the former period there was no uniformity 
anywhere, and the field was broken up into a multitude of minor 
basins of deposition each the theater of an individual sequence 
of events, while during the latter the whole of southwestern Iowa, 
northwestern Missouri, eastern Kansas, and probably an even 
larger area, acted as a unit. The turbulent conditions of the 
earlier period became merged into the uniform conditions of 
the later one. Gradually larger and larger areas came to act 
together and local sequences came to have a wider and wider 
applicability. It is the beds of this intermediate period which 
were recognized as the Middle Coal Measures and in the absence 
of unconformity it will be seen that there is @ prior reason to 
expect a series of beds intermediate in character and position 
between the typical Des Moines and the recognized base of the 
Missourian. All who have written on the subject have recog- 
nized that the Coal Measures mark a continuous sequence of depo- 
sition with only local breaks. Any divisions must be more or less 
arbitrarily established though they may be none the less useful. 
The earliest complete section of the Middle Coal Measures 
published was that of Swallow." A comparison of the plates 
1 Geol. Surv. Missouri, I and II, pp. 82-86, 1855. 
