87 
The following digest of the foregoing table sets forth the 
paleontological grounds of the distinction into different beds. 
The Acervularia and Bryozoa Beds, while each holds more 
than 50 per cent, of species not ranging beyond it, contain 19 
species, or 28 per cent, of the whole, in common. Lithologic¬ 
ally they may be described as a series of dark, bituminous 
limestones and shales; and in the general facies ot the forma¬ 
tion, they present themselves structurally as one mass. Below 
this mass is another of strikingly different aspect, composed of 
pale buff, massive limestones, with little shaly or bituminous 
matter, in which 1 have distinguished the Stromatopora and 
Pleurotomaria Beds; which, while each holds about 38 per 
cent, of peculiar species, contain at the same time 9 species, or 
about 20 per cent, of the whole, in common. Above the Acer¬ 
vularia Beds we find another physical change in the Magne¬ 
sian Beds. These are dark-buff, coarse, rough, vesicular, with 
few fossils. Of these, 3 or 38 per cent, are peculiar; 2 are 
species which have too wide a vertical range for use in strati- 
graphical determinations, and 2 are singularly identified with 
species in the bottom of the formation. The Chert Beds pre¬ 
sent another set of physical characters ; though I suspect the 
amount of Chert is very variable. 
I would suggest then, as the most obvious and tenable 
method of grouping the strata, the following: 
