the boundary. The geology, so far as the hard rocks are con- 
cerned, is as simple south of the line as it is complex north of it. 
Hence, although, as the map shows, the addition of the whole 
of Cohasset and a portion of Scituate much more than doubles 
the area to be described, it extends but slightly the limits of this 
paper, which is still, notwithstanding its title and areal scope, 
chiefly a detailed account of the ledges of Nantasket. 
The numerous islands of Boston Harbor, as any one may ob- 
serve in going from Boston to Nantasket by water, are, with 
unimportant exceptions, composed, like the Nantasket Beach 
area, wholly or almost wholly of rounded drift hills and con- 
necting beaches of sand and gravel; and the fine sections of 
some of the drumlins due to marine erosion show that in most 
cases they are pure drift accumulations, at least above sea level. 
The Boston Dasin does not afford a stronger topographie contrast 
than is presented when the Nantasket steamer, having rounded 
Dumkin Island, the graceful drumlin outline of which is abruptly 
terminated on the west by marine erosion, and followed the 
winding channel past the beautiful drumlins of the World's End 
and Planter' Hill, sweeps by the bold and massive ledges of 
Rocky Neck and Nantasket. We pass in an instant from an 
area which is deeply covered by drift and presents only the most 
typieal drift-contours, to an essentially driftless area, where the 
ice-sheet left hardly sufficient detritus to fill the narrow gorges 
and chasms dividing the rocky hills. 
The problem involved in this unequal distribution of the drift, 
in this abrupt passage from an area in which the ice-sheet acted, 
at least during its later stages, as an important agent of depo- 
sition, to an area in which its action was purely erosive, lies 
beyond the scope of this paper ; although, if the drift had not 
been swept from the Nantasket area, there would have been little 
occasion for a special study of its geologic features. We may 
now simply accept the almost perfect exposures of these rock 
masses as one of the best gifts of the ice-sheet to the student o( 
our loeal geology ; for there is probably no equally limited area 
in the Boston Basin more worthy of detailed and thorough in- 
vestigation than this southeast corner. 
