36 The American Geologist. 
July, 1905 
row Dorchester-West Roxbury syncline ; while along the 
southern border the dip is southerly and the conglomerate 
passes below the slate of the much broader and composite 
Quincy-Milton syncline. That the anticline itself is not a 
simple arch is plainly indicated by the narrow band of slate 
developed at intervals along the middle of the conglomerate 
belt and the existence of at least two anticlinal axes, pitch- 
ing to the east and rising to the west, is further indicated 
by the fact that toward the west, where erosion has cut 
through the conglomerate and interbedded flows of basic 
lava we have exposed, not one, but two, ridges of the under- 
lying crystalline rocks — granite and felsite — representing 
the floor upon which the conglomerate series was deposited. 
These two axes are, at most points, of unequal prominence ; 
and in their denuded western extensions the northern axis 
largely predominates , forming the broad, irregular and 
broken ridge projecting into the Boston basin from the 
western highlands of Dedham and Needham and including 
the granite, quartz porphyry and felsite of the Bellevue 
Hill district and the Stony Brook reservation, the felsites 
of the northern part of Hyde Park and the felsites and 
more basic lavas of the Mattapan district of Dorchester. 
The minor southern axis is seen in the narrow band of fel- 
site and basic lava, lying mainly south of the Neponset, 
between Readville and Milton Lower Mills. 
Over a part of this area several flows of basic lava or 
andesite are interstratified with the conglomerate ; and 
during the geological revolution or period of disturbance 
following the accumulation of these strata and the formerly 
overlying slate upon the old floor of felsite and granite, 
they were forced into a gigantic arch from one to nearly 
three miles broad. This great fold, however, partially 
broke down in the making, and its collapse was attended 
by the formation of the minor folds and numerous faults. 
Subsequent erosion has been so extensive as to remove the 
entire thickness of slate from the crest of the great anti- 
cline, except where it has been carried down most deeply 
by these minor folds and the faults, occurring now in nar- 
row and discontinuous belts wedged in between the larger 
masses of conglomerate. The erosion has also been suffl- 
