352 
The American Geologist „ 
June, 189(? 
closely resembling Gyroceras imdulatam , bat probably differ¬ 
ent. It is not yet described. 
It is not easy to say whether this almost absolute dearth of 
invertebrate fossils is due to an actual scarcity of invertebrate 
life or to the subsequent destruction of shells or other calca¬ 
reous material by infiltrating water through strata almost de¬ 
void of carbonate of lime. It seems wiser to prefer the for¬ 
mer cause, as in the latter case impressions would probably 
be left though the fossils were dissolved, as has happened in 
so many cases elsewhere. Moreover, even the few mollusks 
that remain have lost all their calcareous matter and had 
others existed there would seem to be no reason why they too- 
should not have left at least some trace of their former pres¬ 
ence. The cause of the absence of invertebrates must be 
sought in the conditions of the environment. 
But the almost absolute barrenness of the Erie shales is- 
more puzzling. Long days of labor in this stratum will in all 
probability be rewarded with nothing more than a small 
handful of broken specimens or a few indistinct impressions,, 
and these only in certain stations. This shale, whatever view 
we may take of its relations, is in Cayahoga county at least 
300 feet in thickness. Eastward the mass increases rapidly 
until in the eastern part of the state it reaches at least 2,000 
feet, while westvvardly and southwardly it thins away to a 
feather edge and disappears. To the eastward, moreover, it 
must be further noted that the Black shale is not found and 
in mid Pennsylvania and New York the whole body of the- 
Chemung—the equivalent in part of the Ohio shale—takes on 
the same greenish hue. 
The most natural explanation of these facts is that the- 
Black shale marks the shallow portions of the Appalachian 
gulf where vegetable and animal life was present but where 
no great volume of sediment was brought in from the adjoin¬ 
ing land, while the thicker and barren greenish shale to the 
eastward marks an area of deeper water where sediment was 
abundantly deposited, while at the same time conditions did 
not allow, save locally and temporarily, the formation of. 
limestone as in an open sea. 
Furthermore, it may be noted that the fishes of the Upper 
Devonian of Ohio and those of the same era on the opposite- 
