1890.] 
135 
[Crosby. 
valley, and, feeling then the full friction of the unyielding ledges, 
its progress is necessarily slow, notwithstanding the comparative 
steepness of the slope down which it moves. On the other hand, 
the somewhat uniform distribution of the bowlder clay over the en- 
tire glaciated area shows : (1) that the movement of the great ice- 
sheet, as proved by occasional erratics which were imbedded in the 
ice, greatly exceeded that of the main part of the ground moraine, 
the ice-sheet slipping over the till ; and ( 2 ) that this must have been 
the case because, omitting the results of subglacial drainage and 
the final ablation of the ice, no specially vast accumulations of drift 
occur on or near the southern limit of the ice-sheet, and only very 
limited, local and exposed areas within the glaciated zone have been 
swept bare of drift by the ice action alone. It appears, then, that 
too little attention has heretofore been given to the consideration 
that the movement of the ice-sheet was in some degree analogous 
to that of a great land-slip. In both cases the progress of a some- 
what yielding and mobile mass is facilitated by an underlying clayey 
layer saturated with water. 
DISTINCTION OF GLACIAL AND NON-GLACIAL SILTS. 
The predominance of rock-flour in detritus of glacial origin and 
its paucity in the products of chemical decay and the mechanical 
action of wmter, together with the fact, demonstrable by experiment, 
that a very large proportion of the flour must ultimately be deposited 
with the clay, raise the question as to the possibility of finding here 
a criterion by which to determine whether or not the finer silts of 
different geological ages are of glacial origin. In order to test the 
general fact of the occurrence of rock-flour in the clay beds of the 
Boston Basin, two fresh and typical examples of clay were collected 
at the large brick yards in West Cambridge, at depths of ten and 
twenty -five feet. \ These are smooth and plastic bluish-gray clays, 
seemingly homogeneous, although slightly gritty between the teeth. 
A portion of each sample was first washed and found to contain a 
large proportion of rock-flour, which exhibits under the microscope 
the same' characters as that obtained from the till. The combined 
water was then determined by ignition. No. 1 (10 feet) gave 2.92 
per cent., and No. 2 (25 feet) 4.20 per cent. This indicates that 
the proportions of pure clay can not much exceed one-fourth and 
one-third respectively ; and the large remainder, in each case, must 
certainly be mainly, if not wholly, rock-flour. A slightly oxidized, 
but otherwise similar clay, from a depth of seven feet at the brick 
