178 SURFACE GEOLOGY. 
have been written by several geologists upon these subjects, which can 
be consulted by those desiring further information. It is obvious that 
our first duty is, to state whatever facts have been observed, and then 
discuss the general bearings of the subject, if an opportunity is pre- 
sented. It may be premised, however, that the glacial theory of the 
origin of the cold and of the dispersion of the materials seems to explain 
all the phenomena better than the older view of the agency of icebergs. 
Any complete discussion of the phenomena must relate partly to the 
effects produced upon the ledges by the ice-movement, and partly to a 
description of the materials transported, their position, shape, size, 
amount, etc. We will first speak of the action upon the ledges. The 
ledges have been broken, rounded, or embossed, planed down, smoothed, 
and striated. The formation of pot-holes took place after the ice began 
to melt. 
Fracturep LEpDGEs. 
Since the Helderberg period, the rocks of New England had been 
subjected to sub-aerial decomposition, whereby they were softened and 
rendered friable to great depths, often as low as the water level, or as 
much as a hundred feet. The change was mostly chemical, consisting 
of the removal of the alkalies, and the disappearance of lime and magne- 
sia, by solution, the residue being clayey. Hence the ledges were in 
excellent condition for removal by the ice-sheet. Enormous quantities 
would be easily rubbed off, and then assorted by water. Besides the 
decomposed rocks, the ice removed perhaps as great a mass of the solid 
portions, which now constitute boulders and the pebbles of the till. Still, 
this ice action does. not represent all the erosion that has taken place in 
our state, as may be proved by calculations of the mass that has been 
removed from the ledges to fill up gaps in the strata. It has been gen- 
erally estimated that this amounts to as much as the average height of 
the land above the sea, or 1,200 feet in New Hampshire. The condition 
of the surface in the Southern states, in Brazil, and in certain sheltered 
spots in Minnesota and Massachusetts, as described by Hunt, Hartt, 
White, and others, illustrates the nature of the land surface with us, be- 
fore the ice acted upon it. 
In the earlier New England reports, several cases of ledges fractured 
