MANSFIELD: ROXBURY CONGLOMERATE. 123 
surfaces usually show a polished or glazed appearance. If the clay 
of one stratum be washed and sifted it is found to be composed of 
grains of all shapes, sizes, and weights down to the most impalpable 
flour. The till often contains nests, lenticular layers, and occasional 
thick beds of gravel, grit, sand, and brick clay, frequently curled up 
and contorted as if rolled over upon themselves along with the clay 
in which they are inclosed. Though the stones usually are distrib- 
uted without reference to their relative weight and size, one may 
occasionally observe within a given area large erratics arranged with 
their longer axes lying in the same direction. The same thing has 
been noted with regard to the smaller fragments of grit and clay. 
Stone (p. 30) calls attention to the indiscriminate mixture of coarse 
and fine fragments in the till and adds that the lower layers contain 
more fine material and a much larger proportion of distinctly scratched 
or glaciated stones than the upper layers. A. Geikie notes (p. 548- 
549) that “the detritus is for the most part fresh and angular. Its 
trituration by the glacier reduces the size of the particles but retains 
their angular character, so that, as Daubrée has pointed out, the sand 
that escapes from the end of a glacier appears in sharp, freshly-broken 
grains and not as rounded, water-worn particles.” 
A few years ago Crosby made an investigation of the constituents 
of till taken mostly from the greatest accessible depth in a number of 
drumlins in the vicinity of Boston. Some of his conclusions are out- 
lined as follows: — 
(1) “it is doubtful if stones and boulders more than two inches in 
diameter often form more than five to ten per cent. of the till and the 
instances will certainly be very rare and local where they form more 
than twenty per cent.” (Crosby, j, p. 120). 
(2) “the normal till of the Boston Basin is composed, after the 
larger stones are excluded, of about 25 per cent., or one-fourth, of 
coarse material which may be classed as gravel; about 20 per cent., or 
one-fifth, of sand; 40-45 per cent. of extremely fine sand or rock-flour, 
and less than 12 per cent. of clay” (ibid., p. 123). The gravel particles 
are described as ranging in size from one twelfth of an inch to nearly 
two inches in diameter, having subangular forms, and tending, when not 
of exceptionally hard or brittle character, to present flat forms, smooth 
and often distinctly striated on two or more sides, while the remain- 
ing aspects are quite angular or exhibit little wear (ibid., p. 125). The 
sand consists chiefly of angular quartz grains with fragments of some 
other minerals. The rock flour is composed of still more nearly pure 
quartz and even the clay seems to contain impalpable quartz (ibid., 
p. 127-129). 
