INTRODUCTION. 
21 
through the Appalachian chain, the coarse sandstones and conglomerates 
indicate the close of this period; while the same geognostic line, from 
the northern side of Lake Huron, by the course of the Cincinnati axis, 
quite to the centre of Tennessee and still farther to the south, is marked 
by bands of coral limestone. 
In the region of Eastern New-York, the coarse materials of mechanical 
origin are accompanied by littoral or shallow-sea shells; while farther 
from the shore line and the influences of the stronger currents, the same 
deposit became the habitation of other forms adapted to the changed 
condition, and finally coral reefs occupied the bed of the ocean in the 
vicinity of the present Ohio valley. 
The ocean bed of this geological period, like that of all others and of 
the present epoch, was not uniform in its conditions, nor in the depth of 
sea. There were certain lines of no great variation in depth, along which 
accumulated the forms fitted for the conditions. The most prolific zone in 
the limestone formations is that marked by the most perfect development 
of corals; and it is along this line, also, that other forms accumulate in 
the greatest numbers. Where, on either side of this zone, the conditions 
change, whether it be from deeper water, the deposition of arenaceous 
and argillaceous material, or from whatever cause that corals and bryozoa 
become less numerous, we soon find, likewise, a diminution in the other 
forms of life; until, as we approach the ancient sea margins, or exjdore 
the muddy and sandy bottoms where no limestones occur, we sometimes 
find an abundant fauna adapted to these conditions, and of the kind where 
brachipods are in small numbers, and where corals are scarcely seen. 
We have been accustomed to look to the northeast for the source of the 
sedimentary materials of this group; and to regard some part of the 
present Northern Atlantic ocean bed as having been occupied by land, 
the destruction of which furnished the sedimentary materials for this 
formation. We are scarcely prepared, therefore, for the information which 
comes as the result of investigations in the Canada Survey, that while 
this source may have been to the northeast from us in New-York, and far 
beyond the limits of our explorations, it lies in a direction more to the 
east than we have been accustomed- to believe. 
