210 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
formation, 2,300 feet, may be added to the figures here given, and a 
maximum thickness of about 5,300 feet may be obtained for the sedi- 
ments of the Boston Basin. 
:— Summary of Structure. The foregoing discussion of the 
structure may be summarized as follows:— 
(1) The escarpment forming the northern boundary of the slates 
is suggestive of a fault. No valid arguments have been brought 
against this supposition. 
(2) The age relations of the Somerville slates are indeterminate, 
but the supposition of Carboniferous age accords best with the known 
structural facts. 
(3) The area from Brighton south to Chestnut Hill Reservoir and 
west to Newton Upper Falls is probably a monocline of increasing 
dip toward the north. The Chestnut Hill slate belt marks the line 
of a great overthrust fault with the fault plane dipping gently north. 
(4) The Savin Hill-Brookline conglomerate area forms a broad, 
flat-topped, unsymmetrical anticline. 
(5) The Dorchester-Roslindale slate belt forms a narrow, faulted 
syncline. 
(6) The Hyde Park-Mattapan-Squantum conglomerate zone forms 
a broken and faulted anticline, overturned slightly northward and 
eroded at the western end deeply enough to separate the limbs. 
(7) In the Neponset-Hough’s Neck region the question of age 
again arises in connection with the slate. The structural evidence 
favors the supposition of Carboniferous age. Hough’s Neck may 
represent part of a faulted anticline. 
(8) The presence of arkose, which elsewhere forms a basal mem- 
ber of the series, is opposed to the supposition of a fault along the 
granitic border, as postulated by Crosby. If the arkose is really 
basal and the Neponset slate is Carboniferous, then the fault proba- 
bly occurs under the drift north of the arkose and granite and the 
latter are brought up to the stratigraphic level of the younger sediments. 
(9) The outlying Wellesley-South Natick area is probably a broken 
syncline. ‘The arkosic nature of much of the sediments indicates 
that probably the basal members of the series are exposed. 
(10) The Hingham area affords the only section that has been 
measured. The maximum figure, 1,445 feet, does not include the 
base of the conglomerate nor any of the overlying slate. The struc- 
ture is characterized by folds greatly modified by faults. 
(11) Nantasket is a faulted monoclinal region, with several inter- 
stratified flows of lava and beds of tuff. 
