1890.] 
131 
[Crosby. 
especially in such typical examples as the Convent Hill and East 
Boston drumlins, is a very sharply defined and regular line, except 
where cracks or other lines of' weakness have favored the penetra- 
tion of meteoric waters. 
GLACIAL EROSION AND TRANSPORTATION. 
Classified according to origin, or dynamically, the different kinds 
of detritus composing the till fall into two classes — mechanical 
and chemical. These two classes of agencies, it is true, always 
cooperate, but usually the action is either chiefly mechanical or 
chiefly chemical. It has already been pointed out that there has 
been practically no chemical change in the lower or unoxidized 
till since the close of the ice age, and in the superficial or oxidized 
till the chemical changes consist chiefly of the higher oxidation and 
hydration of the iron, with comparatively little kaolinization. The 
conditions have, however, undoubtedly been far more favorable to 
chemical changes since the disappearance of the ice-sheet than 
during the time when it covered the land. Hence, it would seem, 
we may safely conclude that the ice age was a period when the op- 
eration of the chemical forces at the earth’s surface was virtually 
suspended over the glaciated area, and the mechanical action of the 
ice- sheet and of the water — subglacial and superglacial — which 
accompanied it, reigned supreme. It must be admitted, however, 
that, as already pointed out, the comparative absence of feldspar 
and other silicates in the finest sand and the rock-flour is a sufficient 
indication that some of the clay in the till is of strictly glacial or- 
igin ; and with this must also be included that due to the complete 
crushing of argillaceous rocks. But it is likely that these glacial 
additions to the clay in the till are fully offset by the clay that was 
eliminated through subglacial kneading and drainage, as already 
explained. 
It follows, then, that substantially as much chemical detritus or 
clay as now exists in the till must be referred to preglacial times 
and regarded as representing the soil (chiefly sedentary) which 
covered the glaciated regions before the advent of the ice. The 
preglacial valleys, like those of the present day, must have been 
occupied largely by mechanical and more or less water-worn and 
assorted detritus ; wffiile the uplands, as now in low latitudes, were 
probably covered by residuary soil due to the chemical decay of the 
rocks in situ. Sedentary or residuary soils include not only clay, 
