228 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
observed by the writer and mentioned (page 162) lends support to 
this view. 
Metamorphism. In the northeastern part of the basin the rocks 
display shearing in much the same degree as do those of the Boston 
Basin but there is comparatively little evidence of dynamic meta- 
morphism.. The occurrence of ripple-marks and mud-cracks in the 
Pecunit Street ledges, comparatively undisturbed, indicates the slight 
extent to which these rocks were affected. 
In the southwestern part, on the other hand, a high degree of meta- 
morphism has been attained. The ledges between Walpole and 
East Walpole and also farther southwest show a marked development 
of secondary mica and in some cases of chlorite. Pebbles in the con- 
glomerates are marked with pressure striations and are sometimes 
deformed and indented, while the finer sediments have become highly 
schistose. The contrast in degree of metamorphism between the 
northeastern and southwestern sections may be partly explained by 
their difference in structure; but it is probable too that the south- 
western sediments were under a greater pressure of superincumbent 
strata than was the case with the northeastern sediments, since the 
former are known to be basal while the latter are probably higher 
members of the series. 
Summary. The Norfolk Basin sediments occupy a trough-like 
depression, narrowing toward the northeast. Field evidence shows 
that it is divisible into two areas, which present different types of struc- 
ture, the line of separation being probably a transverse fault, following 
roughly the present valley of the Neponset River south from Green 
Lodge. The southwest area is characterized by closely folded strata, 
overturned southward, the best exposures being seen at Pondville. 
‘Transverse faults are numerous in this area, but longitudinal faults 
are not so clearly indicated. The relations of the arkose to the grani- 
tite show that at Pondville at least the southern boundary of the basin 
is a true sedimentary contact. 
The northeastern area is characterized by monoclinal structure 
with no clear evidence of folding. The northern boundary of the 
basin in this region is believed by Crosby to be sedimentary, the coarse 
so-called basal conglomerates resting directly against their parent 
ledges, and owing their formation to the progressive southward tilting 
induced by the growth of a deep fault on the north side of the Blue 
Hills, during the period of deposition of the sediments. The present 
writer, however, regards the basal arkose exposed. at Pondville as the 
normal base of the sedimentary series of the region and believes that 
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