70 The American Geologist. February, 1904. 
out spurs v/hich flank deep valleys, may be inferred from the 
data of depth given above. Only a long period of subaerial 
exposure and the tear and wear of erosion and corrasion dur- 
ing Palaeozoic times can account for these unequalities of the 
hard "bed rock." The decision of the question put by Darton 
in his paper already referred to, p. 673 or 17th Annual, Part 
II, whether the quartzyte of this region is simply a metamor- 
phosed portion of the water bearing Dakota sandstone under- 
lying the western part of South Dakota and cropping out at the 
foot of the Black Hills, or not, should be the subject of a special 
petrographic study aided by favorable exposures. 
Considering the fact that no member of the sedimentary 
Algonkian has as yet been reported from this region except the 
red Sioux Quartzyte, especial attention must be paid to a pe- 
culiar and exceptional condition revealed by a boring in section 
25 of Delapre township. At that locality a bluish black slate 
was struck at a depth of 45 feet which was found to be so hard 
that the well-borer was about to quit boring, when he, after 
having gone for 90 feet through the slate, struck the red Sioux 
quartzyte and a little before water. Is. this slate, overlying the 
red quartzyte and on the other hand directly overlain by the 
blue till, to be referred to the Algonkian or to a more recent 
age? — possibly the Cretaceous and more especially to the Ben- 
ton shales of the Upper Cretaceous, as might be inferred from 
the usual occurrence of the Cretaceous formation above the 
"bed rock?" The probability of the latter suggestion is how- 
ever much weakened by the consideration that the Benton 
shales are only feebly developed in the northern part of the 
region under discussion and also in the adjacent area to the 
west, as brought out by James E. Todd in U. S. Geological 
Survey, Irrigation paper No. 34, p. 16, where he estimates the 
probable thickness of the deposit in that region to only about 
50 feet. Another circumstance, which may question the pos- 
sibility of a correlation of this slate to a possibly hardened por- 
tion of the Benton shales, is that the occurrence of the latter at 
so high a level would be inconsistent with the usual level of the 
Niobrara, v/hich is far lower, even in the northern slope of the 
trough. On the other hand the suggestion, that we have here 
to deal with another member of the Algonkian associated with 
the Sioux quartzyte, seems to be much favored b}^ the associa- 
