49'2 A. AY. GRABAU PALEOZOIC DELTA DEPOSITS OF NORTH AMERICA 



return to the Rochester region. After a while both the Xiagaran and the 

 Eurypterid faunas became extinct in this region, and the red sedimenta- 

 tion of the Vernon shale set in. That this was intimately connected with 

 the red shale (Longwood) sedimentation farther east can not be ques- 

 tioned, and it seems equally clear that the Pittsford shale sedimentation 

 corresponded in part to the Shawangunk sedimentation farther east, as 

 shown by the similarity of organic remains. 



In Michigan the Vernon shale is absent, or at any rate its earlier 

 phases are represented by salt deposits. As shown by the borings in 

 southeastern Michigan, what appear to be Xiagaran dolomites are di- 

 rectly succeeded by salt deposits, the lowest bed here having a thickness 

 of 160 feet 89 in one section. It may, of course, be true that the limestone 

 at the base still belongs to the Salina, but that seems rather questionable. 

 The shales succeeding and separating the different salt beds are not red, 

 so that it appears that Vernon sedimentation did not extend so far. 



Interpretation of the sections — The Shawangunk conglomerates.— 

 Considering the deposit as a whole, we see that it has roughly the form 

 of a semi-cone, whose greatest thickness is in the region of the present 

 Kittatinny Mountains in Xew Jersey and Pennsylvania. From this point 

 it decreases regularly northeastward and southwestward and, since it is 

 absent in southern Xew York, it must also decrease, probably at a similar 

 rate, toward the northwest. This rate averages 20 feet per mile. South- 

 eastward it seems to decrease at a more rapid rate, as shown by the fact 

 that in less than 24 miles it has decreased from 1,900 to 1,200 feet, or at 

 a rate of about 30 feet per mile. The fact that the formation becomes 

 coarser to the southeast, and that the pebbles are less purely of one kind 

 of material, argues that the source from which they were derived lay in 

 that direction. The same thing is shown by the character of the pebbles 

 themselves, which are mainly vein quartz, and by the abundance of the 

 feldspar, since there is no other rock capable of supplying such material 

 except the crystalline old land on the southeast. Some part of the series 

 might indeed have been derived from the early Siluric and late Ordovicic 

 deposits of a similar character (Tuscarora and Bald Eagle), but the 

 probabilities are that if the eastern folded portions of these strata made 

 any contribution it was a slight one. It must not be forgotten that the 

 center of the Shawangunk deposit is shifted approximately 145 miles to 

 the northeast of the center of the Bald Eagle deposit to a region where 

 the lower series had thinned away to an insignificant sandstone layer. 



89 Royal Oak Well No. 2. See Lane, Geol. Survey of Michigan, vol. v, pi. xl ; also 

 Wyandotte well, pi. xlvi. 



