MANSFIELD: ROXBURY CONGLOMERATE. 199 
by the steep southerly dip of the strata at that locality may be due to 
the fact that the beds in question lie on the north or downthrow side 
of the fault. 
On the supposition of the Carboniferous age of the slate the apparent 
conformable succession of the strata and the occurrence of the con- 
glomerate at Medford are readily explained. The supposed down- 
faulting of the slates offers no difficulty, since in that case the effect 
would be simply to bring higher beds of the series in opposition to 
lower beds. The evidence in favor of either Cambrian or Carbon- 
iferous age cannot be said to be conclusive, but the latter hypothesis 
accords better with the structural data now at hand, and has therefore 
been represented in the accompanying section (Plate 6). 
——:—The Brighton-Newton Area. According to Crosby the 
Brighton Newton area forms an anticline relatively symmetrical at the 
western end where the melaphyr extending eastward forms the axis. 
Toward the east the anticline is broken down and becomes a faulted 
monocline. “The southward thrust of the conglomerate arch has crushed 
and partly concealed the narrow syncline of slate along the southern 
border. According to H. G. Woodward no less than six beds of mela- 
phyr, ranging in thickness from 25 to 200 feet, and one important tuff 
bed, besides several dikes of melaphyr, are included in the area 
(Crosby, g, p. 12). 
Burr, on the other hand, finds no evidence of anticlinal structure 
in this area. The conglomerates south of the melaphyr dip north- 
ward at prevailingly low angles, while north of the melaphyr the dips. 
are still in the same direction, at somewhat steeper angles. Thus the 
whole forms a monocline of increasing steepness towards the north. 
Furthermore, he finds no repetition such as might be expected in the 
case of anticlinal structure. The conglomerates, with interbedded 
sandstones and slates, grow progressively finer northward and there 
is no swinging of strikes at the eastern end of the conglomerate such 
as might be expected were the strata arched (Burr, b, p. 64-65). _ 
The writer’s observations have not been extended throughout this 
entire field but in traverses both north and south of the melaphyr 
region from Brighton to Newton Upper Falls he has found the dip 
of the conglomerate prevailingly northward and the strike fairly uni- 
form, with local variations only, in a northeast-southwest or nearly 
east-west direction. 
South of the monoclinal area occurs a narrow slate belt which is 
exposed in several localities near the eastern part of the Chestnut Hill 
Reservoir, on the north side of Beacon Street at Newton Center and 
7 
