1890 .] 
125 
[Crosby. 
ino- that this is the case to such an extent that the clay beds may 
be regarded as embracing fully one-half of the original composi- 
tion of the till. It follows, therefore, that the clay beds, though 
so low-lying and inconspicuous, must be equal in volume to the 
coarser modified drift, and hence that they must in this region 
have been deposited very largety beyond the present margin of the 
land. The smallness of the proportion of pure clay in the till be- 
comes, however, more marked when we take account of the large 
stones and bowlders ; and it would probably be within bounds to 
say that the till in its natural condition is often less than one-tenth 
and rarely more than one-eighth pure clay. All this, of course, 
exalts the importance of the rock -flour, which, although so unob- 
trusive, must now be recognized as the specially abundant and 
distinctive feature of the till, being approximately equal to all the 
coarser materials taken together, and nearly four times as abun- 
dant as the pure clay. Before proceeding to discuss the bearing 
of this important fact upon the problem of glacial erosion and the 
distinction of glacial and non-glacial silts we may profitably observe 
more closely and in greater detail the characters of the various 
constituents of the till. 
DETAILS OF THE ANALYSES. 
Gravel . — Although the material included here ranges in size from 
one-twelfth of an inch to nearly tv?o inches, it is all essentially 
alike in presenting subangular and more or less distinctly glaciated 
forms. An occasional well-rouilded pebble of quartzite or almost 
equally flinty felsite may be observed which has evidently been 
detached from the conglomerate of this vicinity and has suffered 
but little subsequent wear ; but these are practically the only ex- 
ceptions to the rule that the fragments and grains are subangular 
and tend, when not of an exceptionally hard or brittle nature, like 
quartz or feldspar, to present flat forms, being smooth and often 
distinctly striated on two or more sides, while the remaining aspects 
are quite angular or exhibit but little wear. It has been a constant 
surprise to find the smaller particles, especially of the softer kinds 
of rock, almost equally with the larger rock fragments and the 
great stones or bowlders of the till, testifying to the rubbing, grind- 
ing and striating action of the ice-sheet. These observations raise 
an interesting question as to the way in which the till was moved 
