1879.] 223 (Upham. 
rough and angular form of its boulders, which are often from five to 
ten and sometimes twenty or thirty feet in diameter. This forma- 
tion, called upper till, covers the surface of New England generally, 
except where it is overlain by the modified drift, which was deposited 
slightly later by the floods discharged from the melting ice in its far- 
ther retreat northward. 
The third and lowest division of the drift is called lower till, and 
appears to be a ground-moraine, accumulated beneath the moving 
yce-sheet. This deposit forms the principal mass of the hills alluded 
to as occurring in the vicinity of this city. It is distinguished by its 
very compact and hard structure; by its clayey detritus, which is 
usually dark colored and sometimes bluish, because of the imper- 
fectly oxidized state of its iron; and by the planed sides of its 
boulders and pebbles, which are often marked with striae, and are 
generally not so numerous and large as in the upper till. Both these 
divisions of the till are entirely unstratified, boulders, pebbles, sand, 
and clay being indiscriminately mixed; but an obscure lamination in 
planes parallel to the surface is often noticeable in the lower till, 
especially where a section has been for a short time exposed to the 
weather. This was due probably to the mode of its accumulation, 
and seems to show that the ice, in its passage, added new material to 
the surface of its ground-moraine, which lay undisturbed below. The 
hardness of this deposit, which requires it to be loosened with a pick | 
before it can be shovelled, appears to have resulted from the immense 4 
pressure of the ice. i: 
In many portions of New England this lower till, or ground- \ 
moraine of the ice-sheet, is heaped in prominent hills, which are i 
“very abundant upon certain limited areas, extending ten to thirty 
miles and five to ten miles wide, while in adjoining districts of 
equal extent none of these accumulations are found. ‘Their develop- 
ment in Boston, in neighboring towns, and in the islands of the har- 
bor, is as typical and striking and as well adapted for a study of 
their character and origin, as in any region where they have been 
observed. Your attention has already been directed to this subject 
by papers which appear in the Proceedings of this Society, first by 
Professor Shaler in Vol. x11, 196-203, and again by Professor Hitch- 
cock in Vol. x1x, 63-67. ‘The present essay gives a more detailed 
description than has previously been presented of these hills as ex- 
hibited in Boston and its vicinity, and will attempt to show the con- | 
ditions and causes of their accumulation, so far as they seem to be r 
discerned, pointing out also the problems connected with this and \ 
their remarkable distribution, which remain to be solved. 
= = SSS 
