Crosby.] 
132 
[May 21, 
Avkich is a true chemical product, but also quartz and iron oxides 
in both coarse and fine grains which have been set free by the de- 
composition of granitic rocks, gneisses, schists, etc., as well as 
partially decomposed particles of mica and other silicates and more 
or less rounded residual masses of the various rocks — chemically 
formed pebbles and bowlders or “hard heads.” Of all these vari- 
ous forms of preglacial detritus, the clay is the only one that can 
be even approximately identified in the till. We have already as- 
sumed that the losses which the clay suffered during the ice age 
were compensated by the glacial additions, and we may perhaps 
make the farther assumption that at least an equal amount of coarser 
detritus may be referred with the clay to a preglacial origin. This 
means that possibly as much as one-fourth and quite certainly not 
more than one-third of the detritus composing the till of the Bos- 
ton Basin was in existence before the ice age, and that the remain- 
ing two-thirds or three-fourths must be attributed to the mechanical 
action of the ice-sheet and its accompanying torrents of water, in 
other words, if we assume the average thickness of the drift as 
thirty feet, the amount of glacial erosion can scarcely fall below 
twenty feet. After scraping aAvay the residuary clays and half-de- 
composed material, the ice-sheet has cut more than an equal depth 
into the solid rocks. 
It is with no little hesitation that I state this conclusion, con- 
troverting, as it does, so large a part of what has been written on 
the subject of glacial erosion, in recent years. I have held with 
many others that the ice-sheet had but little erosive power and that 
in its progress over the land it did little more than to sweep away 
the preglacial soil and smooth and striate the underlying ledges. 
It is important to notice here, however, a possible source of error 
in the analyses not yet referred to. It has been convenient here- 
tofore to regard all the clay in the till as fully formed chemically, 
and containing the normal proportion of combined water. But a 
perfect mechanical analysis of the till, unvitiated by the rock-flour, 
if that were possible, would undoubtedly show a somewhat larger 
proportion of clay than that indicated by the chemical test ; for, 
since kaolinization is a progressive change, the preglacial residuary 
soils, like those of the present day, must have passed gradually 
downward into the underlying rocks. The kneading action of the 
ice-sheet would render this rotten rock still more clay -like, and hence 
an appreciable if not a considerable part of the clay in the till is 
