MODIFIED DRIFT IN NEW HAMPSHIRE, 5 
blocks and fragments which fall from the bordering cliffs are carried along 
on the surface of the ice, or are contained in its mass with others torn 
from the rocks over which it moves, and under its vast weight these act 
as graving tools to round and striate the ledges beneath. The similar 
striation of all northern countries, and the formation of the till, are proba- 
bly due to a similar cause, namely, a moving ice-sheet which overspread 
the continents from the north. 
This continental glacier had accumulated sufficiently deep to cover 
every mountain summit in New Hampshire. That it overtopped Mount 
Washington is fully proved by recent discoveries of the state geologist.* 
Its thickness farther to the north was so much greater than in this lati- 
tude that its immense weight caused the ice to flow slowly outward. 
The direction of its current in New England was between south and 
south-east. Its terminal front in the United States coincided nearly with 
the course of the Missouri and Ohio rivers, passing into the ocean south 
of Long Island. Its greater extent east of the Missouri resulted from the 
increased snow-fall of this side of the continent. The termination of this 
ice-sheet in the Atlantic, south-east of New England, was probably like 
the great ice-wall bordering the Antarctic continent, along which Sir J. 
C. Ross sailed four hundred and fifty miles, finding only one point low 
enough to allow the upper surface of the ice to be seen from the mast- 
head. Here it was a smooth plain of snowy whiteness, extending as far 
as the eye could see. The Humboldt glacier, in Greenland, discovered 
by Dr. Kane, is sixty miles wide where it enters the sea, above which it 
rises in cliffs three hundred feet high. All icebergs have their origin 
from glaciers which thus extend into the ocean, being broken off, because 
of their lower specific gravity, by the uplifting power of the water. 
Cause of the Arctic Climate. The conditions which brought on the 
severe climate of this epoch have been the subject of much speculation 
and discussion. A theory which, with much probability, refers the ice- 
sheet to an astronomical cause, and claims to determine the date and 
duration of the glacial period, was proposed by James Croll in 1864, and 
has been advocated by James Geikie in his recent work on the Great Ice 
Age. The earth’s path about the sun is not exactly a circle, but is a 
* See Chapter II of this volume. 
