578 Surface Geology of Eastern Massachusetts. (October, | 
the whole, a closer correspondence with geology than the reliefs 
shaped by the action of ice. This results from the essential un- 
likeness in modes of action of the agents in question. The topo- 
graphic work done by rivers is effected mainly through degrada- 
tion. 
“Time but the impression deeper makes, 
As streams their channels deeper wear.” 
The rivers of this section seldom deposit much material along 
their courses, and the detritus delivered at their mouths is 
usually transported to a greater or less distance, and finally 
laid down by the sea; so that this process, although the opposite 
of degrading, is, more marine than fluvial. The action of the 
sea in modifying the contour of the land, on the contrary, 
is, generally speaking, twofold: degradation, which, when the 
waves work upon rocky shores, is determined more or less in 
direction and amount by the geologic structure of the de- 
graded land; and building up, which operates by the transpor- 
tation of loose materials and their accumulation at particular 
points, and depends mainly upon the direction and force of 
ocean currents, tidal waves, and prevailing winds, and upon the 
general shape of the land and ocean bed, though quite inde- 
pendent of their geological constitution, except mediately in 80 
far as this has determined their form. In this region, suc 
features as the outer end of Cape Cod, portions of Nantucket, 
the southeastern part of Martha’s Vineyard, part of the coast 
of Essex County north of Cape Ann, and Lynn, Nantasket, and 
Duxbury beaches appear to be due to the constructive action of 
the sea; and of course they can be expected to conform in trend 
only imperfectly, if at all, with the features produced by deg- 
radation. The close correspondence between their forms and 
structure usually observable in those topographic details fash- 
‘ioned by the degrading action of the sea or rivers is, NO doubt, 
largely due to the mobility of the water. It is by virtue of this 
wonderful property that the ocean possesses a power of discrimi- 
nation, — a sort of tactual sense, as it were, — which enables it 
to act differently upon rocks having unlike constitutions, 
thus, in effect, to dissect a rocky coast in a manner ana” i 
to the unlocking of the molecular structure of a mass of ps : 
-a transmitted beam of light. In striking contrast with sla 
the action of a large land glacier, such as exists at ye oe 
Greenland and Antarctic Land, and probably spread over ; a 
England at the time when the phenomena— drift and stria 
and 
