— mg er M oM 
MANSFIELD: ROXBURY CONGLOMERATE. 229 
it was probably formerly continuous or contemporaneous with the 
arkose north of the Blue Hills and at Dedham. He believes that the 
absence of the giant conglomerate along the north base of the hills 
is not due to concealment by a fault but either to erosion or to non- 
deposition, the true basal sediments being the arkoses there exposed. 
He considers that the northern boundary of the basin is a fault, with 
downthrow to the south, along the southern flank of the Blue Hills. 
He regards the giant conglomerate as either an uneroded remnant of 
a former more extensive mass now preserved by down-faulting, or as 
a local deposit rearranged by aqueous agencies and accumulated 
along the base of a fault scarp that developed progressively during 
the period of deposition of the sediments, the fractured and broken 
Masses of conglomerate and the igneous impregnations bearing witness 
to the successive slips of the growing displacement. As regards the 
Southern boundary, the writer agrees with Crosby that a deep fault 
with downthrow on the north appears to be demanded by the apparent 
monoclinal structure. On the supposition that the arkoses form the 
real base of the series, and that the fault which permitted the forma- 
tion of the giant conglomerates began during the period of deposition, 
the sediments of the eastern portion of the basin are probably more 
recent than part, at least, of the Pondville series. 
The occurrence of igneous impregnations in the conglomerate lends 
Support to the view that igneous rocks play a more important part 
in the history of the basin than is generally supposed. 
The rocks of the eastern part of the basin are relatively unaltered, 
-while in those of the southwest portion a high degree of metamorphism 
has been attained. 
Tar NARRAGANSETT Basın.— Literature. The Narragansett Basin 
has been the subject of a considerable literature, of which a bibli- 
ography may be found in Professor Woodworth’s report (d, p. 212- 
214). The writer has not attempted any general study of this 
Material but has confined his attention to the monograph entitled, 
Geology of the Narragansett Basin, the joint work of Shaler, 
Woodworth, and Foerste, and to a few lesser papers. 
_ Shape and Size of the Basin. The Narragansett Basin, as shown 
In Figure 8, is a somewhat irregular trough extending north from the 
Coast of Rhode Island into Massachusetts, where it bends east towards 
the Atlantic near Duxbury, Scituate, and Cohasset, and comes within 
about six miles of the sea (Shaler, et al., p. 7). The eastward bend is 
made near the northeast corner of the state of Rhode Island and in 
