186 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
railway cut near Canton Junction actual plant stems have been recov- 
ered, as noted on page 101. Ni VOR Re 
——:—Local Unconformities. No cases of local unconformity have 
been seen by the writer in the Norfolk Basin. There is however some 
evidence of contemporancous erosion here as in the Boston Basin. 
Occasional pebbles of sandstone and grit similar to the sediments of 
the basin are inclosed in the conglomerate. An instance of this kind 
is found in the exposures south of Bear Hill (Dedham VII, E F 18). 
Relations to Subjacent Rocks, The coarse conglomerate along the 
south side of the Blue Hill Range is believed by Crosby to be the 
basal member of the series and to rest in sedimentary contact against 
the crystallines of the Blue Hills region (n, p. 477). There is however 
some reason for questioning this view. The discussion of the evidence 
for and against Crosby's idea is reserved for the following chapter. 
At Pondville the basal strata are unquestionably exposed. Arkose, 
overlaid by slate, sandstone, and quartz-pebble conglomerate, rests 
against the granite from which it was derived. The actual contact 
of the arkose with the granite is not seen but the arkose is so evidently 
the recomposed debris of the granite that there can be no doubt that 
the latter forms the floor on which the former was deposited. The 
formation of the arkose and of the quartz-pebble conglomerate above 
indicate a long period of subaerial disintegration of the land from 
which these materials were derived. 
Little or no resemblance is to be seen between the rocks exposed 
near the granite at Pondville and the glacial accumulations of the 
Great Ice Age. Since the actual contact is not exposed it cannot be 
determined by direct observation whether polished and striated sur- 
faces underlie the arkose but the presence of the latter and the over- 
lying stratified deposits, both entirely unlike ordinary till, render any 
such supposition untenable. Similarly there is no evidence that 
striated surfaces underlie the conglomerate along the south side of 
the Blue Hills. 
Summary of the Norfolk Basin Series. (1) In the northeastern part 
of the basin the conglomerate lies along the northern border of the 
sediments. In the southwestern part conglomerate appears in most 
of the localities where the sediments are exposed. 
(2) Igneous rocks, so far as their occurrence is known, are sub- 
ordinate features. 
(3) In the northeastern part of the basin the gradation in texture 
of the series from coarse to fine is well marked from north to south. 
In the southwestern part a less well-defined gradation in the opposite 
direction occurs. 
