648 TE fOORNALROLMGHOE OG NE 
from 1:6 to 1:30. The greater number fall between 1:10 and 
1:16, so that the average may be taken as lying between these 
figures. 
In connection with these basins usually a considerable 
number of small slips or true faults are found. These rarely 
have a great throw though in a number of instances they paral- 
lel each other, forming a step fault with an aggregate throw of 
considerable extent. 
Frequently these slips parallel the basins. In some cases 
cracks or veins filled with calcite also run in a parallel direction. 
Another peculiarity often observed is that the slips do not run 
Fic. I. 
clear through the coal; more usually they come in from the top 
but disappear before reaching the bottom. This, however, is not 
confined to the most pronounced basins, but it is also frequently 
found in those coals which have a true bedded character, such 
as the Mystic coal. 
In discussing the origin of these basins it will be convenient 
to consider separately the lines now formed by the top and the 
bottom of the coal. Considering first the latter the resemblance 
to the cross section of a valley is at once seen to be striking. 
This is well brought out in the figure of the Vanderberg mine, a 
small slope about three miles south of Pella on the Des Moines 
River. Fig. 2 represents the conditions at this point. The Saint 
Louis limestone here forms an almost continuous outcrop some 
distance along the river. It rises to a height of thirty or forty 
feet. At the place in question the coal lies undisturbed ata 
level at least fifteen feet below two contiguous limestone out- 
crops only a few feet distant. It is evident that this coal was 
formed in a depression in the limestone, and the relations exhib- 
ited on the ground show just as clearly that this was a basin of 
erosion. There is no evidence of disturbance or faulting. The 
