PAPERS ON GEOGRAPHY AND GEOLOGY 211 



There is no reason to believe that these cavities are 

 anything but sinks ; every geologist who has seen them 

 agrees to that. The only other possible explanation is 

 that they are pot-holes, but their shape disposes of that 

 possibility without argument. Their age can be deter- 

 mined from the deposits that rill them; all g I sjists who 

 have examined them believe that the green shale or clay 

 and sandstone is Pennsylvanian in age. but the age of 

 the black laminated shale is in doubt. An authority to 

 whom the material was shown has suggested that it may 

 be Devonian: another, that it may be Pennsylvanian: but 

 neither of these has yet had opportunity to study the 

 deposits or the plant fossils sufficiently to make a defi- 

 nite statement. The color of the shale and the occur- 

 rence of Devonian shale in cavities in the Xiagaran lime- 

 stone near Chicago tend to strengthen the possibility 

 of Devonian age. If it proves to be Pennsylvanian, the 

 marked difference in character of the two shales and 

 their abrupt contact where they occur together are ample 

 proof of two distinct epochs of deposition. 



Two hypotheses may be offered to explain the condi- 

 tions under which the sinks were filled by shale. One is 

 that after the sinks were formed the outlet at the bottom 

 of each became choked and the pits gradually rilled up 

 with material washed into the resulting ponds by surface 

 run-off, in which may have been included much organic 

 debris. The possibility that some of the debris is the 

 residue of plants growing in the sink itself is untenable 

 because the shale is in horizontal layers as is found only 

 in subaqueous deposits, shows no old soil or peat or coal, 

 and contains no plant remnants in place. The other 

 hypothesis is that the area was completely submerged be- 

 neath the sea, and there filled with silt. sand, and organic 

 debris. The regularity of the deposits seems to indicate 

 the latter hypothesis the more probable, in which case the 

 abundance of plant remnants indicates that the area 

 was near the shore of that sea. 



It is safe to state that at some time after the deposi- 

 tion of the Niagaran limestone or dolomite in the Silur- 

 ian sea over this area and preceding either the Devonian 

 or the Pennsylvanian period, there was relative erner- 



