306 The American Naturalist. [April, 
green shale beds. It thus appears that the lithological character 
of these beds changes toward the south. 
The question as to the cause of the peculiar brecciated char- 
acter of the limestone in Iowa and adjacent parts of Illinois 
presents a pertinent field of inquiry. Worthen and Hall make | 
frequent mention of this feature of the St. Louis limestone, 
and White goes so far as to say that “during the time of 
the deposition of this limestone there seems to have been 
some slight disturbance of the strata, apparently amounting 
only to local disarrangements of its own layers. This is prin- 
cipally shown in the upper division, and consists of the 
slipping, bending or slight distortion of all the strata, also 
by the breaking up-of that limestone into angular fragments 
which have in many cases become recemented together by similar 
limestone material, forming the breccia above referred to. The 
most of the disturbance seems to have prevailed during the de- 
position of the upper division.” * It would be exceedingly inter- 
esting to learn the nature of these local disturbances. Hall 
speaks of it as follows: It “ consists generally of a breccia com- 
posed of fine, compact, ash-colored limestone in fragments of 
various sizes, having the interstices filled with a subcrystalline, 
yellowish, granular, calcareous material, which is sometimes quite 
pulverulent, and rarely very compact. The rock at Keokuk, and 
at points above this on the river, as well as at Mt. Pleasant and 
elsewhere, appears like the attenuated margin of a more important 
formation, presenting the usual fractures of the thinning out of a 
limestone, viz., a brecciated and concretionary structure. This 
presumption proves to be true, for as we trace the rock south- 
ward beyond the state, it presents other aspects, gradually losing 
its concretionary and brecciated character, and becoming a more 
important limestone formation.” ? 
This explanation can hardly be considered adequate, for it 
would necessarily follow that the attenuated margins of all lime- 
stones should present the same characteristics; whereas they do 
not. That the brecciated character is a marginal attendant in 
1 Geological Survey of Iowa 1870, Vol. I., p. 218, 
2 Geological Survey Iowa, 1858, Vol. I., Part 1, p. 98. 
