196 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
of the Charles River between Cambridge and Brighton. The Som- 
erville portion, though locally much folded (ibid., p. 77), is, on the whole, 
tilted southward. It is matched on the south side of the Charles 
River by a band of slate or mixed slate and conglomerate, varying 
in width and dipping north and overlying a great mass of northward 
dipping conglomerate. According to this view the Medford conglom- 
erate underlies the slate and rises from beneath it. The northern 
outcrops of the slate are stratigraphically older and lower than the 
southern outcrops and are more arenaceous. The structure of the 
eastern portion of the Somerville area is a relatively simple mono- 
cline with a low southerly dip. The western portion, however, 15 
much more complicated and appears to consist of an assemblage of 
folds not sufficiently exposed for correlation (ibid., p. 38-39). An 
anticlinal axis with a northeast-southwest trend is believed to pass 
beneath Meetinghouse Hill in Watertown. This has been represented 
in the accompanying section (Plate 6). Assuming an average dip 
of 20° for the northern limb of the great syncline, the thickness of the 
slates in Somerville may attain the maximum of 2,300 feet. A well- 
boring in Cambridgeport which does not pass entirely through the 
slate shows the latter to be at least 900 feet in thickness (ibid., p. 77): 
On the south slope of College Hill in Somerville (ibid., p. 25), there 
is a limited outcrop of quartzite. Similar rocks occur at Everett 
and East Chelsea, though the identity of these quartzites has not been 
established. In Somerville the quartzite lies in the axis of a pinched 
syncline. It is not known whether these beds overlie or come within 
the Somerville slate but no stratigraphically higher rocks are seen 
overlying the quartzite in any of these localities. At Malden (ibid., 
p- 70-72), along the railroad between Faulkner and Maplewood, 
cross-bedded slates occur with a northwesterly dip of 55°-90°. The 
indications from the cross-bedding are that the strata have been 
overturned toward the southeast. 
The age of the slates has never been satisfactorily determined. 
They are probably either Carboniferous or Cambrian. The argu- 
ment for their Cambrian age is set forth by LaForge (ibid., p 81-82) 
as follows: — 
(a) They are lithologically similar to the recognized Cambrian 
slates of the region. 
(b) They are lithologically similar to the slate at Hyde Park, which» 
though of undetermined age, is certainly older than the conglomerate, 
since it is cut by granite, pebbles of which occur in the conglomerate: 
(c) They are cut in one or two places by intrusions which are’ 
