206 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
West of the conglomerate, arkose occurs along the southern bor- 
der. No special significance appears to be attached to the arkose by 
Crosby other than that it shows that the granitic rocks of the Blue 
Hills area were subject to erosion during the accumulation of the sedi- 
ments (ibid., p. 479). He regards the arkose, together with the over- 
lying slate, as upper members of the Carboniferous series. It may be 
remarked, however, that arkose in other parts of the Boston Basin 
and in the Norfolk and Narragansett Basins is more associated with 
the basal beds of the series. At Pondville, in the Norfolk Basin, for 
example, the arkose rests upon the granitite and is overlaid by shales, 
conglomerates and sandstones. The formation of arkose requires 
certain climatic conditions that will be discussed later. These con- 
ditions would doubtless obtain not only over the combined areas of 
the several basins under consideration but probably also over far 
wider regions, so that deposits of arkose in the several basins might 
be regarded as in a measure synchronous. Since the arkose is usually 
associated with the basal sediments of the southern basins it is probable 
that the same is true of the Boston Basin also, and that the arkose 
along the southern border really represents the basal member of the 
series. This supposition is opposed to the view that a fault passes 
between the granite and the arkose. More probably it passes to the 
north through the slate somewhere beneath the drift. East of the 
conglomerate the evidence is uncertain. So far as can be judged the 
rock is all slate under a deep drift cover. Crosby believes that the 
northern part of this area is referable to the Carboniferous series 
and the southern part to the Cambrian. He therefore produces his 
fault almost due east from the northern side of President’s Hill to the 
coast, in order to obtain the desired separation. In the accompanying 
section (Plate 6) the fault has been indicated along the granitic border 
in accordance with Crosby’s idea, but on account of the probably 
basal character of the arkose the writer has more recently formed the 
opinion that it should be placed north of the arkose at least, and per- 
haps north of some of the slate also. 
——:—Outlying Areas: —W ellesley-South Natick. From the vicin- 
ity of Newton Lower Falls southwestward through South Natick, scat- 
tering outcrops of conglomerate, slate, and melaphyr indicate that the 
Carboniferous rocks of the Boston Basin are extended in that direc- 
tion. The most remote outcrop is southeast of Farm Pond in Sher- 
born, reported by Tilton. The area is largely drift covered. Many 
of the ledges have been intensely metamorphosed and recognition 
of their true character is uncertain. The general structure of the 
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