MANSFIELD: ROXBURY CONGLOMERATE. 155 
The cement of the conglomerate is usually silicious or felsitic, sel- 
dom argillaceous. In the cases where the cement is felsitic the rock 
is composed largely of that material; pebbles of quartzite, felsite, and 
granite, together with grains of quartz, occur embedded in a fairly 
homogeneous ground mass of felsitic material. The felsitic cement 
is particularly noticeable in localities near which felsite occurs in situ. 
In the vicinity of Mattapan and Hyde Park, for example, the felsitic 
element in the conglomerate becomes more and more abundant so 
that there seems to be a gradation between undoubted conglomerate 
on the one hand through felsitic conglomerate and felsite breccia to 
felsite on the other. Similar phenomena are noted at Medford, in 
parts of Newton, South Natick, and Nantasket. The whole succes- 
sion is apparently similar to that described by Dutton in his account 
of the volcanic conglomerates of the High Plateaus, as noted on 
page 116. There the formerly clastic matrix of the conglomerate 
has been metamorphosed into homogeneous material entirely similar 
to the contained fragments (loc. cit.). A peculiar conglomerate that 
may be referable to the same type occurs north of President’s Hill 
in Quincy (Boston IX, T 33). The matrix seems to be felsitic and 
the included fragments entirely granitic. In one specimen there 
were streaks that appeared to be microgranitic. In some places 
the rock resembles an indurated tuff. Specimens from this locality 
have been examined petrographically by T. G. White, who states 
that they look exceedingly like volcanic tuff with laths of plagio- 
clase and stubby hornblendes. He says that unless the rock is in 
reality a true volcanic that has in some way enveloped the rounded 
pebbles we have a conglomerate surprisingly metamorphosed and 
recrystallized in the presence of neighboring igneous rocks (T. G. 
White, p. 124). 
:— Pebbles. An examination of the pebbles of the conglomer- 
ate shows that on the whole felsite in many varieties is the substance 
most abundantly represented, with quartzite and granite next in 
importance. Out of 65 specimens collected from all parts of the 
basin 50 were noted as containing felsite while 38 contained quartzite, 
and 25 granite. Other rocks much less frequently represented are 
slate, melaphyr, contemporaneous sandstone and grit, white quartz, 
and diorite. 
The size of the pebbles is usually variable. In most localities the 
pebbles or fragments grade down to the grains of the matrix without 
any well-marked line of separation. In Medford the pebbles do not 
ordinarily exceed three inc: es in diameter. North of the narrow 
Slate belt that passes through Chestnut Hill Reservoir the pebbles 
