1872.) | 81 [Perry. 
sions, and existing under kindred relations. We should naturally 
look for the denudation of the surface of the underlying solid rocks, 
with their polishing and striation, all taking place at once, and in 
directions determined by the course of each glacier, in fact by that 
of its several different portions. Loose rocks might be taken up, 
debris gathered from the sides of the valleys, forming lateral lines of 
angular stones and half-comminuted matter, while some of these 
would unite in medial ridges on the occasional junction of two or 
- more ice-streams. Resting on inclined surfaces, pressed onward by 
forces suited to secure motion, accelerated and retarded by the alter- 
nation of favorable and of adverse conditions, the mass under the 
action of cold would encroach all winter long upon the plains below, 
and during each recurring summer, as heat prevailed, it might, and 
no doubt usually did, retreat somewhat for a while, leaving where the 
ice melted terminal moraines, with lateral and medial, as witnesses of. 
the advance of the preceding season. In case of a great increase in 
the intensity of the cold, the mass of ice must be augmented, the old 
markings planed off, new polishing effected, fresh inscriptions 
written, while the previous morainic masses would be wiped away 
by the advancing stream of ice, and thus few, if any, traces left of 
their former existence. 
Such is a brief, and of course a very inadequate account of what 
was, perhaps, the beginning of the vast ice-sheet which eventually 
spread over all this regions While it is, indeed, only a rough out- 
line, it possibly serves to call to mind substantially what we may 
suppose was the opening chapter in the history of the Glacial period. 
The inception of the work was the formation of a great number 
of limited glaciers of distinct ice-streams on the flanks of the moun- 
_ tain heights, in the northern half of the continent, and still later in 
all New England. This beginning was not, of course, exactly syn- 
chronous in its several different parts; the work travelled from the 
north southward; and when it was only fairly initiated in what is now 
our neighborhood, there was probably a far more advanced stage of 
progress in the northern portions of America. It is scarcely nec- 
essary to add, after what has been presented, that in the very nature 
of the case the records are extremely scanty. The whole account 
must be constructed largely on theoretical grounds; theoretical, not 
as entirely apart from a stable foundation, but in the sense that, hav- 
ing few immediate facts before us, we are compelled to rear the 
structure, not without facts, but in the light of such as stand, if I 
PROCEEDINGS B. 8S. N. H. — VOL. XV. 6 SEPTEMBER, 1872. 
