76 Dr. F. V. Hayden on the Primordial Sandstone 
crest, we have the Potsdam sandstone, then immediately above 
it with remarkable uniformity, a series of beds of limestones, 
containing true Carboniferous fossils. We infer, therefore, that 
both northward and southward the same uniformity of geologt 
eal structure continued unless we have evidence to the contrary. 
The observations of Dr. J.8. Newberry render it quite pro 
Just above the Potsdam (?) is a limestone filled with corals, 
apparently Cheetetes lycoperdon or rather the same with that s0 
eommon in the Trenton with branching stems, formerly included 
in C. lycoperdon, but evidently distinet. On the mountains bound- 
ing the Colorado basin the Carboniferous rocks rest directly up- 
on the granite.” 
We have now considered the Potsdam sandstone in its geo 
hical extension over the West as far as we are acquainted 
with its existence, and have pointed out the localities where iti8 
revealed. Along the Mississippi Valley and eastward, most im 
portant discoveries are made annually, which show it to be de 
veloped every where, when the conditions are favorable for it 
exhibition. 3 It is true that in some localities beds of more —_— 
‘age repose directly upon Azoic rocks, but in these cases ma 
fhe Briniordish-eandeianoats match 7 
researches of Dr. B. F. Shumard in Texas, have shown that the 
