115 
[Crosby. 
General Meeting, May 21, 1890. 
The President, Prof. F. W. Putnam, in the chair. 
The following papers were read : 
COMPOSITION OF THE TILL OR BOWLDER-CLAY. 
BY W. O. CROSBY. 
Introduction. 
While studying the surface geology of the Boston Basin I have 
been greatly impressed by the vast amount of stratified sand and 
gravel forming the numerous plains and terraces as well as the wind- 
ing ridges or kames. The unmodified drift or till of this region 
occurs mainly in the form of smoothly rounded hills or drumlins ; 
and the intervening areas are covered almost continuously and often 
to a considerable depth by these accumulations of modified drift. 
Except, however, that the till assumes the drumlin contours here 
to an unusual extent, the Boston Basin may, in the character and 
distribution of the drift deposits, be fairly regarded as representa- 
tive of the low-hing or valley portions of the glaciated regions 
generally; and just as truly as the unmodified drift covering any 
area has been in large part transported from the northward by the 
ice-sheet, the modified drift has been in large part transported from 
more elevated and h^nce more inland areas, irrespective of the di- 
rection, by water ; and it may, in consequence, enclose rock frag- 
ments of very different lithological character from any observed in 
the adjacent or subjacent till. 
But, whatever its source or exact mode of transportation and 
deposition, there can be no question but that practically all the great 
volume of modified drift encumbering such an area as the Boston 
Basin has resulted from the washing, assorting and stratification of 
the till by water, partly, perhaps, while the ice-sheet still covered 
the land, but mainly during the final melting and recession of the 
ice, when the valleys must have been flooded with water and the 
country dotted with temporary lakes. The appreciation of this fact 
naturally raises the question as to what has become of the clay with 
which all of this coarse detritus was once incorporated. The term 
bowlder-clay denotes a heterogeneous mixture of clay with sand, 
