LOO The American Geologist. August, i89a 
no details were given nor any references made to the evidence 
this appears to be the first notice calling attention to the ex- 
istence of a physical break in the Carboniferous rocks of the 
M ississippi basin. 
A decade later, White,* calling attention to the same fact, 
stated that another unconformity existed between the St. Louis 
limestone and the underlying rocks of the Lower Carbonifer- 
ous. These remarks also appeared subsequently in the Iowa 
Report. f hut were very general in their character. Yet there 
is added to Hall's observation the important tact that the St. 
Louis limestone also overlaps in the state of Iowa. 
In Iowa, at least, it appears that the extension of the Coal 
Measures beyond the boundaries of the St. Louis limestone 
was much more than an overlap in the ordinary sense of the 
word, such as might have taken place off shore in gradually 
deepening waters. It was a sinking of an ancient land sur- 
face which had been more or less profoundly carved into hills 
and vales, affording protected nooks favorable to swamp for- 
mation and the rapid accumulation of vegetable materials. 
Since the observations recorded by Hall and White were 
originally published little work bearing upon the subject in 
hand has been done until quite recently. In Missouri, Iowa 
and western Illinois much valuable information has accumu- 
lated within the past few T years so that now the principal 
changes of the coastal contour which took place during the 
Carboniferous over the region mentioned are tolerably well 
understood. The exact character of the unconformities re- 
ferred to by the -writers named have been made out with con- 
siderable detail ; while numerous minor physical breaks have 
also been recognized. 
The problem therefore centers around the oscillation of the 
Old Carboniferous shore-line in the upper Mississippi basin. 
The changes are graphically shown in the accompanying dia- 
gram, (Figure 1), representing the movements of the land 
margins in the probable direction of greatest shifting. 
The Devonian rocks doubtless extended northward beyond 
the present limits of Iowa. Toward the close of the Devonian 
period the seas over this region gradually contracted. This. 
*Am. Jour. Sci., (2), vol. xlv, pp. 331-334, New Haven, 1868. 
tGeology Iowa, vol. I, pp. 225-229. Des Moines, 1870. 
