326 SURFACE GEOLOGY, 
until the removal of the eccentricity of the orbit restored the warmer 
conditions. 
Two suggestions respecting the probable origin of all the inter-glacial 
deposits found in New Hampshire will not be out of place. 1. An ex- 
planation of Prof. Torell, of Sweden, is adequate to account for the 
example underneath the lenticular hill in New Ipswich,* and any other 
stratified beds in similar position. He represents that as the glacier 
commenced to exist and to move extensively, there must accumulate in 
favorable localities many stratified glacial deposits, so that a geological 
section of the edge of the ice would present (a) pre-glacial beds; (d) 
stratified glacial deposits; (c) a ground moraine; (d) the ice with its 
terminal moraine. Hence the advancing glacier may often cover strati- 
fied sandy deposits not of inter-glacial origin, 2. Our sections indicate 
that the stratified beds commonly occur between the lower and upper 
tills, as in Figs. 29, 32, 34, and 36. The Champlain fossils occupy the 
same position in the Portland section (Fig. 59). Assuming the correct- 
ness of our views regarding the origin of the two tills, all the inter- 
glacial phenomena are beautifully explained. It is not to be presumed 
that no variations in the position of the edge of the ice-sheet existed. 
The outer limit must have varied very much from time to time, just as 
it does at the present day in existing glaciers. When the ice retreated 
a few thousand feet, its melting would give rise to currents transport- 
ing sand. A change in temperature, or other conditions causing a re- 
advance of the ice-sheet, would cause the stratified beds to be covered 
again, and the mass might even push a short distance over marine 
deposits at the ocean border. But no ground moraine made its appear- 
ance with this reddvance. No deposits were left behind except the 
débris contained in the ice itself, or upon its surface; and this fell to 
the ground during the melting process, and now remains as the upper 
till. This distinction between the tills has been lately recognized by 
English and Scotch authors. It also appears that the British inter-gla- 
cial beds occupy the same position with ours between the two kinds of 
till,t so that probably our explanation of the New Hampshire beds would 
* This is situated just south-west of Jeft’s hill. By the side of the road an excavation shows about five feet of 
sand beneath the glacial drift, probably the lower till. The total thickness of the sand is not seen. In the over- 
lying drift I observed a boulder of porphyritic gneiss four feet in length. Between this point and New Ipswich 
village the road cuts through a sand containing boulders, which is also probably underneath the glaciated till. 
Tt ‘ The till or bouldereclay that rests above these intercalated beds usually differs from the stony clay which 
