482 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
the railroad. Its place of oiisin as a glacial block must have been 
still farther northwest, and if we may reason upon analogy with 
the ])0uldcrs in the Narragansett Basin, then it seems pro})able 
that this was one of the boulders in the Roxbury conglomerate 
somewhere in the Boston Basin. If this be correct, and there 
seems to be nothing to the contrary, it must be recognized that 
the source of the Oholus boulders was sufficiently widespread to 
have supplied the Carboniferous conglomerates of both the Narra- 
gansett and the Boston Basins. Walcott early suggested that 
those occurring in the Narragansett Basin might have been 
derived from the Upper Cambrian l)cds of Newfoundland. This, 
taken literally, is improbable, but it is extremely likely that there 
was a more or less continuous outcrop of Upper Cambrian quartz- 
ite extending from Newfoundland southward which originated 
as a sandstone on the flanks of Appalachia. This quartzite 
probably lay off to the east of our present shore-line, for not only 
has no trace of it been found south of Newfoundland, as would be 
expected had it lain to the west of the shore-line, but that position 
is more in accord with the fact that the boulders occur almost 
exclusively in the eastern parts of the basins. Whether the 
sediments of the Narragansett and the Boston Basins w^re ac- 
cumulated centripetall}^, as hinted by Woodworth, or were all 
introduced in essentially one direction, has little or no bearing 
upon this matter, unless similar boulders should be found in the 
western parts of the basins. 
3. ASPIDELLA-LIKE MARKINGS FROM THE CAMBRIDGE 
SLATE. 
It is fortunate for us that the drift is not absolutely continuous 
over the country around Boston, for then we should be dependent 
upon drift boulders for our knowledge of the bed-rock. As it is 
we seldom have recourse to the character of the drift in attempting 
to understand the distribution of the solid rock beneath. There 
is so much that we are uncertain about, however, that any evi- 
dence we may gain from the drift is welcome. The limits within 
which such evidence is admissible as applying to specific known 
outcrops should be recognized before it is used. Granting a 
lithological similarity between the erratics and the supposed 
parent outcrop, then the drift should be in the path of the ice 
