1879. ] The Formation of Cape Cod. 497 
coarse gravel, three feet; ferruginous sand, one foot, changing 
above into a former perar soil one pa thick; overlain by 
three feet of dune sand, which forms the present surface ninety- 
eight feet above the sea. The highest part of the bank is 
midway between this point and the light-house. On each side of 
the section here noted the shell-beds gradually thin out and dis- 
appear at a distance of about forty feet. The bed below the layer 
of Serpulz contains about thirty-five species, which make up a 
faunal group of distinctly southern character, all of them being 
now found living on the southern shores of New England, but 
several having their northern limit at Cape Cod. The species of 
Serpula is also of southern range, reaching from this limit to 
North Carolina. The upper shell-bed has about the same num- 
ber of species as the lower, but only thirteen are common to both. 
The new species brought in by the upper bed are mostly of 
northern range; though all of these are found as far south as 
Massachusetts bay, several of them have their southern limit here 
or on the south coast of New England. From a comparison of 
these groups, Prof. Verrill estimates that the average temperature 
of the sea at this place had been lowered 15° between the times 
in which they lived. The layer of coarse gravel which occurs 
here at the height of ninety feet, is continuous for a half mile 
from this point, both to the north and south, varying from three to 
eight feet in thickness. About half of its rock-fragments are 
rounded, these being of all sizes up to one foot through; the 
rest, which are rough and angular, range up to two feet and rarely 
to four feet in diameter. This bed has its greatest thickness and 
is coarsest at the highest portion of the bluff, where it closely 
resembles till. The old surface of black soil and the present sur- 
face of dune sand are also continuous along the same distance, 
the latter at the light-house and northward being one and a half 
feet thick. An eighth of a mile south from the shell-beds the 
bluff falls to a hollow about sixty feet above the sea, and in this 
depression the blackened layer becomes a bed of peat two feet 
thick, containing numerous stumps and roots of trees and cov- 
ered by two feet of sand. The rocky stratum, the old surface 
soil and the overlying sand cap the bluff for more than a mile, in 
which its height falls from one hundred and five feet at the mid- 
dle to about thirty-five fect at each end. Below the rocky layer 
it consists of fine modified drift and pre-glacial beds. The his- _ : 
