MANSFIELD: ROXBURY CONGLOMERATE. 241 
granite area must then have had relatively a much greater elevation 
than now, in order to give sufficient grade for the transportation of 
such coarse material so great a distance. Similarly, the presence 
of equally large pebbles of quartzite at the Attleboro locality, together 
with the noted increase in the size and abundance of these pebbles 
southward, indicate that the land in that direction must also have had 
a considerable relief while the Dighton Conglomerates were forming. 
While it is thus seen that the land must have had some diversity 
of form at the time when the latter part of the Carboniferous record 
in the Narragansett Basin was being made, some of the lower mem- 
bers of the series indicate that such was not the case throughout the 
deposition period. The basal arkose with the overlying quartz-peb- 
ble conglomerate and finer beds indicate that previous to the deposi- 
tion of the sediments, and probably also during the early part of that 
period, the country was without great differences in relief, else the 
arkosic material would not have formed in sufficient quantity to per- 
mit its later accumulation in the present beds. 
Question of Original Basins. In his earlier studies Crosby stated 
vigorously his view that the ‘Basins probably existed as such before 
the deposition of the sediments which they contain” (Crosby, b, p. 
181). Similar views were advocated by Shaler and Foerste. ‘The 
former regarded them as “erosion troughs which became the seats 
of excessive deposition,” which “brought about the lowering of the 
Surface in relation to the original bedding” (Shaler et al., p. 13). 
Shaler, however, differed from Crosby’s early view in regarding the 
ancient erosion troughs as far more extensive than the present basins. 
He considered the Narragansett Basin as a broad trough penetrating 
far into the land and possibly including the Worcester trough (ibid., 
P- 9), while Crosby considered the former basins as practically of 
the same area as those'of the present (b, p. 181). Foerste also adopted 
the idea of pre-existing basins, for he wrote of the Precarboniferous 
floor as being “partly above water and furnishing material for arkose 
and conglomerate” (Shaler et al., p. 376). In his later work Crosby 
has made radical changes in his ideas on this subject, for in his paper 
on the Blue Hills Complex he states that it is improbable that the 
basins of the existing Carboniferous rocks were even outlined before 
deposition began (n, p. 464). 
The argument for the pre-existing basins seems to be based chiefly 
on analogy but Shaler calls attention to the structure of the Carbon- 
Werous rocks of the Narragansett Basin — close folds, somewhat 
Overturned toward the borders, and open symmetrical folds within — 
