Some Geological Problems — Calvin. 35 
age, and it is in such ravines that the outlying patches chiefly 
occur. The conditions would be most favorable for the protec- 
tion of the soft sandstone strata, at least from the agents that 
operated during the glacial period, when the ravine occupied by 
the strata was comparatively narrow and had a direction at 
right angles to the flow of the ice sheet. The largest masses 
and most extensive area of outlying Coal-Measures occur along 
the Mississippi, between Buffalo and Muscatine. Between 
these points the river runs from east to west. Was there an 
old Mississippi occupying the same channel practically in pre- 
Carboniferous times? Were these great masses of shales and 
sandstones laid down in a valley of erosion, and have they been 
preserved from denudation because the valley had a direction at 
right angles to the ice flow when glacial conditions and glacial 
erasion were at their culmination? These questions may be 
answered aflirmatively or negatively by some one who has time 
and facilities for working out the problem.* 
A sentence or two in the fifth volume of the Geological Sur- 
vey of Illinois, page 223, would seem to have some bearing on 
the question under discussion . The authors of the chapter on 
the Geology of Rock Island county, Messrs. Worthen and Shaw, 
say: 'There are also some brown beds near Andalusia that 
contain numerous Gasteropods and Orthoceratites, and a few 
miles below, these are overlaid by from eight to ten feet of a 
brown magnesian limestone that contains casts of a large 
Spirifer \ike S. parriianus and Strophomena demissa. These 
brown beds are directly overlaid near the mouth of Stouecoal 
creek by the sandstones and shales of the coal-measures." 
Near Andalusia then it would seem that we have essentially 
the same geological phenomena as in the region about the 
mouth of Pine creek, with this difference, that the sandstone 
containing casts oi Spirifera 2)arrjjana { S. cajxix HaW,) is re- 
presented by a magnesian limestone. A magnesian limestone 
as all know, does not preserve calcareous structures, and so in 
the Devonian dolomite near Andalusia, as in the Niagara and 
other dolomitic limestones of the northwest, the fossil brachio- 
*For particulars relating to the distribution of outliers of the Carbon- 
iferous age, the reader is referred to Hall's Geology of Iowa, vol. i, part 
1, pp. 120-133; White's Geology of owa, pp. 228-9; Geological Survey of 
Illinois, vol. v, pp. 228-232. 
