MANSFIELD: ROXBURY CONGLOMERATE. 214 
(12) The Harbor Islands are composed of slate and appear to form 
parts of a great syncline. 
(13) The thickness of the conglomerate series, together with the 
overlying slates, probably amounts to as much as 5,300 feet. 
(14) The disposition of the conglomerate and slate areas on the 
map (Plate 7) shows that the arms of slate widen and the conglom- 
erate masses grow narrow toward the east, indicating a general 
structure of anticlinal and synclinal folds pitching east, so that in the 
western part of the area the lower members of the series are exposed, 
while in the eastern part the highest beds appear. 
Relations to the Melaphyrs. In several rather widely scattered 
areas of the Boston Basin melaphyr accompanies the conglomerate 
series. The main localities are at Nantasket, Hingham, Hough’s 
Neck, Brighton, Brookline, and the Newtons. It has already been 
noted (page 202) that rocks in the Neponset valley assigned to 
the melaphyrs have been shown to be less basic than that type. 
At Nantasket and Hingham the lavas are generally believed to be 
flows interbedded with the conglomerate series. At the former 
locality no less than five beds of melaphyr and tuff and one of 
porphyrite have been made out by Crosby, as indicated in a previous 
paragraph. The occurrence of the tuff, though limited, is excellent 
evidence of the effusive character of the rock and this is supplemented 
by the occurrence of numerous pebbles of the basic lavas in the con- 
glomerates that overlie the melaphyrs. 
At Hingham the melaphyr mass is believed to represent a com- 
posite of several flows, on account of the distribution of the scoria- 
ceous and amygdaloidal matter. According to Crosby the contact 
of the melaphyr with the arenaceous conglomerate is parallel with 
the strike of the conglomerate, with only minute irregularities. The 
melaphyr does not penetrate the conglomerate more than the latter 
does the former. Irregular cracks in the lava are filled with fine sand 
now highly ferruginous. ‘There are no distinct inclusions of the sedi- 
ments in the melaphyr but a few small pebbles of the melaphyr occur 
in the conglomerate near the contact. The conglomerate and sand- 
stone exhibit no special alteration or induration. It is evident that 
the sedimentary rocks were deposited on the melaphyr for they fill 
cracks in it and are partly made up of debris from it. Near the con- 
tact the melaphyr fragments are large and angular but the conglom- 
erate as a whole is composed chiefly of felsite and granite (Crosby, 
ove: 200-214). 
The relations of the melaphyr to the conglomerate at Hough’s Neck 
