226 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
northwest corner of Great Pond. It might be expected also that 
there would be some evidence of igneous action along the line of a 
deep and growing fault. Evidence of this kind is not entirely wanting, 
for the writer found among the conglomerates on the southern slope of 
Bear Hill igneous impregnations quite similar to those noted at Wal- 
nut Hill and elsewhere in the Boston Basin (see page 157). 
Furthermore, it might be presumed that some representatives of 
the Blue Hills rocks would occur on the north side of the range, if 
the latter were exposed to erosion on the south side. Since the fault 
scarp faced south, however, and the northern slope may be supposed 
to have been more gentle, it would not be expected that so great a 
proportion of these rocks would appear in the conglomerates. Care- 
ful search among the pebbles of the latter on the north side of the 
ridge has thus far failed to produce any types that have been definitely 
assigned to the Blue Hills porphyry, but it has been shown (page 
168) that specimens, collected by H. J. Wiswell from the Roxbury 
Conglomerate in the vicinity of the Bird Street Station, bear a strik- 
ing resemblance to certain facies of the rock in question. Under 
this supposition, then, the Blue Hills region may be conceived as a 
fault block of the Great Basin type, that was uplifted during the 
deposition of the Carboniferous rocks thereabouts and perhaps partly 
or entirely buried by subsequent accumulations of sediment. 
The relations of the southwestern part of the basin’to the eastern 
portion are not clear. From the limited exposures it appears that 
intense folding has occurred, followed by transverse faulting. These 
features are especially well shown at Pondville. The difference in 
the appearance of the strata of the eastern section seen in the rocky 
ridge east of the Neponset River and the rocks of the southwestern 
section seen at East Walpole is so striking that it suggests a line of 
separation corresponding in direction to the transverse faults noted 
farther southwest. These faults at Pondville occurred later than 
the folding. It is possible, therefore, that previous to this transverse 
faulting the eastern portion of the basin may not have been an area 
of deposition and that the sediments now found in that region are 
younger than those farther southwest. The evidence, however, is 
insufficient to warrant any definite conclusions to that effect. 
Thickness. In the eastern portion of the area the section afforded 
by the rocky ridge east of the Neponset River shows no apparent 
repetition by folding or faulting and there is almost continuous ex- 
posure of the rocks for more than a mile. According to Crosby’s 
figures the aggregate thickness of the series is at least 5,000 feet and 
