1872.] TOM : [Perry. 
Boston Harbor to-day. Iam accordingly disposed to recognize the 
blue clay in this instance, at least until further evidence is found, as a 
deposit made in water by no means deep, the localities containing the 
so-called pelagic remains being simply an arm of the sea. Thus, 
while an elevation followed by a depression is perhaps conceivable, 
furnishing another considerable element of time, I am indisposed’to | 
accept it without more satisfactory proof. 
With the Saxicava clays and sands the case is somewhat different, 
or, at least, there need be no special doubt in regard to the main 
points connected with their deposition. They probabiy took their 
places when the region was nearly at a stand-still, or, it may be, in 
connection with a stand-still at a little lower level and a gradual ele- 
vation which followed. The thickness of the beds shows that the 
period of their formation must have been of considerable length. 
But the estimate of its duration must be greatly enhanced by the fact 
that the beds contain littoral shells in great profusion, and that those 
holding such organic remains were accordingly laid down within 
the sweep of the tide. One of these beds, which occurs in 
Swanton, Vt., at about 120 feet above the ocean, is some eighteen 
feet in thickness, and composed almost entirely of shells belonging to 
mollusks which live only along the shore. It therefore must have 
been a very long time in forming, for the shells are mostly perfect, 
while there are many indications that the animals lived and laid 
down their calcareous coverings where they are now found, slowly 
building up the shore for countless generations, as it stood still while 
the waters rose above it, and afterward as they retired, perhaps itself 
rose insensibly from beneath the sea. But this is only one instance, 
and at a given level. There are many similar cases at greatly varying 
heights through a range of some four hundred and sixty feet, at each 
one of which the land was long enough at a stand-still, for more or less 
extensive beds of littoral shells to be laid down. Now, making the 
shortest reasonable estimate of the time occupied in the deposition of 
the Leda clay,—remembering that the Saxicava beds are a shore de- 
posit,—that thus two of these beds at different heights could not have 
been ordinarily formed at the same time,—that such beds in great num- 
bers were deposited, one after another, in long succession between the 
existing sea level and the height of some four hundred and sixty feet,— 
and that there are beneath these marine terraces other deposits of a 
fresh water origin, which must have been a long time in forming,— 
we can scarcely escape the conviction that the Upper Plistocene beds 
afford elements indicative of a vast lapse of time. 
