Crosby.] 
116 
[May 21, 
gravel and large stones or bowlders, and the general aspect of a fresh 
section usually gives the impression that the c\ay is a prominent 
if not a principal constituent. 
If the unmodified drift or till is approximately one-half clay, then 
there must be somewhere in this vicinity beds of Quaternary clay 
equal in volume to the stratified sand and gravel. But this appears 
at first sight not to be the case. The clay washed down by the 
glacial torrents would naturally be deposited only in the bottoms of 
lakes through which the streams flowed, or in the broad estuaries 
through which in this region they reached the sea, or in the margi- 
nal portions of the sea itself. The clay beds of the Boston Basin, so 
far as they are now exposed to observation, are practically limited 
to the low and often marshy lands fringing the shore of Boston 
Harbor and the tidal portions of the principal tributary streams, 
such as the Malden, Mystic, Charles and Neponset Rivers. It 
would be easy, however, to underestimate the clay areas through 
merely superficial observations, because broad, level expanses of 
clay are sometimes covered continuously by a thin layer (five to ten 
feet) of sand, as in the valleys of the Mystic and Charles and to 
some extent over the large and almost completely enclosed area of 
clay holding Fresh and Spy Ponds and drained by Ale wife Brook. 
This bed is the seat of an important brick-making industry ; and 
the clay attains here the unusual elevation of rather more than ten 
feet above mean tide. It is very evident that the main part of the 
clay in these coastal deposits lies below sea-level and the beds 
have been proved by borings to have a thickness in some cases of 
100 to at least 150 feet. But after all reasonable allowances have 
been made the fact still remains that the volume of the clay beds 
of this region is almost insignificant compared with that of the 
coarser kinds of modified drift. It is, of course, probable that a 
large part of the clay once mixed with the sand and gravel of the 
Boston Basin has been deposited beyond the present shore line, in 
Boston Harbor and Massachusetts Bay. On the other hand, we 
have the important consideration tha f this clay has been derived 
from a much wider area than the Boston Basin. Inland, the clay 
beds are few and very limited, the stratified sand and gravel usually 
filling the valleys and resting so far as we know upon till or the 
solid ledges. Taking a broad view, the finest silt or clay washed 
from the till of an entire basin or river system must of necessity 
be gathered by the confluent streams and deposited mainly in the 
