MANSFIELD: ROXBURY CONGLOMERATE. 221 
Crosby concludes that before the deposition of the latter the quartz 
porphyry was deeply oxidized, wholly at the surface and along the 
joints farther down, and that the superficial oxidized portion was 
worn away to provide material for the conglomerate, down to the 
mottled zone (one of the stages noted in the alteration of the por- 
phyry), which was covered and thus protected from further oxidiza- 
tion (ibid., p. 475). According to this view the giant conglomerate 
forms the basal member of the Norfolk Basin series and rests in sedi- 
mentary contact upon the Blue Hills porphyry, from which it was 
largely derived. 
The Streamside Ledge has not been visited by the writer but he 
has penetrated westward from the reference point named, certainly 
as far as the distance indicated by Crosby, though he cannot be sure 
that he has visited the identical outcrop. The south slope of Bear 
Hill down to the stream is strewn with boulders and masses of rock 
of varying sizes up to twenty feet or more in their greatest dimensions. 
Masses of conglomerate thirty feet in length appear nearly flush with 
the slope of the hill, but with so many boulders of varying size all 
about, there seems to be no certainty that evem these large masses 
are in place, though some of them appear to be. The coarseness 
of the conglomerate is probably all that Crosby claims for it, though 
the largest boulders noted by the writer did not much exceed two 
feet in diameter, and these were largely subangular. A careful search 
Was made among the pebbles of the rock for material resembling the 
Blue Hills porphyry, but with the exception’ of a single small and 
doubtful mass no such rock was found and no outcrop showing the 
contact of the conglomerate with any of the granitic rocks of the region 
Was seen. On the other hand the prevailing rock among the pebbles 
IS a fine grained granite, with a pinkish color and unlike the ledges 
and specimens of the Blue Hills prophyry studied by the writer. 
Moreover, many of the felsitic rocks of the Boston Basin are repre- 
Sented both in the pebbles and in the matrix of the conglomerate at 
this place, and, in addition, dark porphyritic rocks resembling some 
facies of the amygdaloidal melaphyr are represented. In one case a 
Composite boulder contained a large boulder of granite and a basic 
Scoriaceous pebble with phenocrysts of feldspar near the margin. 
Among the large conglomerate masses near the northwest end of 
Great Pond and in a single low-lying ledge about a quarter of a mile 
Southeast of Ponkapoag Pond (Dedham IV, Y 27) some boulders 
resembling the Blue Hills porphyry were found by the writer in the 
Conglomerate, but samples collected and compared with numerous 
