MANSFIELD: ROXBURY CONGLOMERATE. 209 
Sula is composed almost entirely of rugged ledges of conglomerate 
and melaphyr that form a striking contrast to the smooth and gentle 
outlines of the drumlin and beach formations. Crosby considers the 
whole area a monocline, which, though shattered by numerous faults 
and dikes, exhibit low and uniform dips. Along the north margin 
of Atlantic Hill at Rocky Neck the dip is south-southeast 10°-20°. 
Southward it diminishes and becomes more easterly. The sedimentary 
rocks with their included beds and granitic floor are broken by longi- 
tudinal and transverse faults into blocks extremely variable in size 
and form. The principal longitudinal faults such as the border of 
the granite from Straits Pond to Weir River Bay have the downthrow 
to the north. The displacement is so great that although the rocks 
dip toward the granite, one passes from older to newer rocks in going 
northward. The conglomerate is so homogeneous and the tuff so 
local that one must depend largely on the lithological characteristics 
of the interbedded lavas in correlating adjacent ledges as well as the 
more remote (Crosby, k, p. 23). The Nantasket section shows the 
granitic floor overlain by six beds of conglomerate alternating with 
five sheets of melaphyr and tuff, with one sheet of porphyrite, the whole 
aggregating 600 to 960 feet in thickness (ibid., p. 24). The tuff beds 
are local. The lava flows are clearly composite, in’ some cases repre- 
senting several eruptions in quick succession. This is conspicuously 
true of the great sheet forming Atlantic and Center Hills. The faults 
are either approximately east-west or north-south and the dikes are 
grouped in three systems: (1) the oldest, N 75-80 E; (2) S 75-80 E; 
(3) the newest, N S (ibid., pp. 25-26). 
:——:—The Harbor Islands. Lovell’s, Gallup’s, George’s, and 
Great Brewster Islands are drift covered. ‘The others, almost driftless, 
form part of a great synclinal fold of slate, with intrusive diabase; 
Calf Island is on the north side of the synclinal and the Brewsters 
are on the south side (Crosby, g, p. 1). 
——:—Thickness: The only place in the Boston Basin where any- 
thing like a complete section has been measured is at Hingham. Crosby 
regards the strata there from lowest conglomerate to highest slate 
as one series. The maximum thickness as given in the above table 
is 1,445 feet, but neither the thickness of the basal conglomerate nor 
of the upper slate is known. According to Crosby’s view the maxi- 
mum thickness is certainly 2,000 feet and if the Nantasket beds are 
included it is probably as much as 3,000 feet (l p. 266). If the 
Somerville slates are considered as Carboniferous and as upper mem- 
bers of the series, then the maximum thickness already noted for that 
