850 
The American Geologist. 
June, 189& 
He is accustomed to find new and strange forms of life sud¬ 
denly appearing on the theatre being, having no evident 
connection with the fauna before them and in like manner 
leaving no discernible traces in the fauna that followed them. 
Illustrations of this principle might readily be found and 
quoted. They indicate gaps in the line of development, local 
breaks in the chain of life, unsolved problems in evolution. 
No evolutionist, however, believes for a single moment that 
any real break occurs in the series or that the genetic connec¬ 
tion has ever been interrupted. He prefers to attribute the- 
apparent gap to the imperfection of the geological record 
whose deficiencies are only too well known. Whole volumes 
of this record are as yet unfound and others have no doubt 
been completely destroyed. But in spite of loss and ignorance 
the induction is so vast that to adopt the opposite conclusion 
would be illogical. 
The placodermic fauna of the Upper Devonian Black shale 
consists of about ten species of Dinichthys , four or five of 
Titanichthys , one each of Brontichthys and Gorgonichthys , 
with Coccosteus , Mylostoma and a few other less known forms. 
Their great number and large size are sufficient to render them 
remarkable, but the suddenness of their appearance and dis¬ 
appearance is yet more striking. In both these respects they 
parallel the great fishes of the Old Red sandstone of Scotland 
which the labors of Hugh Miller have rendered classic. 
The Cleveland shale is a comparatively thin stratum, seldom 
exceeding fifty feet in thickness in northern Ohio. It is black 
and very smooth and soft. But embedded in it are numerous 
concretions in which the fossils occur. These concretions are 
somewhat calcareous, very hard and heavily charged with 
iron pyrites. The fossils themselves are loaded with the same 
mineral, not by its substitution for the original phosphate of 
lime but by its deposition in the Haversian canals of the bone 
which it entirely fills. Under the microscope thin sections 
show the translucent bony structure and the perfectly opaque 
pyritous filling with perfect distinctness. 
The various species are not indiscriminately scattered even 
through this thin stratum. Their distribution, generally 
speaking, is nearly as shown below, at least in the district 
around Berea where they are best known. 
