1872.] 139 | [Perry. 
of rivers, flowing upon typical drift, and occasionally laying down 
terraces along their course, which were in most instances cut through 
in due time by the streams that formed them. In this way, no 
doubt, many ponds and lakes came into existence. And their great 
prevalence must have been a striking feature of the region, at the 
time in question. They probably originated in great abundance, on 
the wasting of the ice-mass, and were associated with the deposition 
of terraces; and they evidently continued into the period now under 
consideration, and became one of its marked characterjstics. They 
would be very likely to appear in the depressions of the drift, espec- 
ially when deposits of clay were first laid down as a lining fitted to 
form an impervious bottom. 
It is not probable that animals peculiar to the time at once made 
their appearance. Some preparation was needful. The ponds 
which had been in existence during the terrace period and continued 
on into the one following, if inhabited while the cold was still con- 
siderable, would be suited to different forms of life, as a temperate 
climate began to prevail. At last, however, as the record tells us, 
creatures came which were of a kind adapted to the changed condi- 
tions, and prepared to bear their part in the work of the period. 
Thus appearing in due time upon the stage of action, they finally 
swarmed in all the ponds and lakes of New England. And they 
have left an evidence of their existence in the individual structures 
they reared—structures which were at once their houses, and a 
genuine portion of themselves,—and in the extensive deposit which 
was largely formed of them. Reference is made to the fresh-water 
mollusks, the testaceous remains of which are now found in the beds 
that were once filled with water, and tenanted by these living crea- 
tures. These mollusks belone principally to the genera, Cyclas, 
Lymnza, Paludina, Physa, Planorbis and Pupa; while the marl beds, 
which are made up of their calcareous coverings, are from a few 
inches to eighteen feet in thickness. 
As marl was perhaps the predominant deposit of the time now 
under consideration ; as it prevails more or less extensively, in places, 
over nearly all New Eneland; as it was laid down not only above 
the Drift but also above the Newer Plistocene beds, in many locali- 
ties in which these occur; as it is thus a marked feature of the older 
Holocene portion of the Post-Tertiary era, it may be perhaps with 
propriety termed the Marl period; for marl is the most abundant 
and characteristic formation now known as a product of those times. 
