202 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
graphically higher position than the conglomerates on either side, 
and that they are caught in a pinched and faulted syncline that is 
slightly overturned northward. This interpretation has been repre- 
sented on the accompanying section (Plate 6). 
——:— The Hyde Park-Mattapan-Squantum Area. From Hyde 
Park east-northeast to Squantum there is a zone of conglomerate with 
oceasional included sandy and slaty zones and masses of basic lava. 
The dips are prevailingly southerly at relatively high angles, 60°-70°, 
or even higher. Toward the southwest the area is split by a mass of 
felsite with irregular boundaries that projects northeastward from Ded- 
ham and Hyde Park. Crosby believes the general structure of this 
zone to be anticlinal with the western part eroded and the limbs sepa- 
rated by the underlying granite and felsite (g, p. 7). The flow struc- 
ture of the felsite originally horizontal is now everywhere highly inclined 
and chiefly vertical, showing that the plication of the Carboniferous 
sediments was shared by their volcanic floor (Crosby, p, p. 80). Ac- 
cording to his view the great arch is not a simple fold, however, for 
it is complicated by several sharp synclines and is broken by numerous 
important faults. The slate is preserved only in narrow belts wedged 
between larger masses of conglomerate by the minor folds and faults. 
He believes also that there are at least three beds of melaphyr and one 
important bed of porphyrite interstratified with the series (g, p. 7)- 
Later studies by the same writer and Miss Bascom have shown that 
some of the lavas formerly classed as melaphyr are really less basic 
than that type (Crosby, p, p. 40). The anticline narrows eastward 
and the axis pitches east (ibid., p. 36). 
The present writer has not studied this complicated region with 
sufficient care to enable him to verify or disprove Crosby’s view of the 
details of structure but the general view of the region as an anticline 
seems to be warranted by the structure of the slate belt immedi- 
ately to the north and by the direction of variation in texture, as 
already noted. The prevailing southerly direction of the dip shows 
that the strata have been almost isoclined and slightly overturned 
toward the north, as indicated on the accompanying generalized 
section (Plate 6). The felsite area is there represented as involved 
in the anticlinal structure, in accordance with Crosby’s view above 
noted. As regards the complications of the anticline it may be said 
that the statement about the preservation of the slate above quoted 
seems a little sweeping, for in a description of the series at Milton 
Lower Mills Crosby speaks of numerous alternations of conglom- 
erate, sandstone, and slate (g, p. 8). That minor folding and faulting 
