MANSFIELD: ROXBURY CONGLOMERATE. 161 
percentage of feldspar and a greater development of schistosity and 
secondary minerals. 
The size of the grains is variable up to about one-quarter of an inch 
in diameter and the shape also varies but is usually angular or sub- 
angular. Of fifteen specimens examined only two had grains at all 
rounded. No definite arrangement was noted; sometimes larger 
grains are scattered through finer material, or again, grains of vary- 
ing size are huddled closely together. 
The cement is usually silicious but sometimes it is felsitic, as in the 
case of the Roxbury Conglomerate. Where the matrix is schistose 
the cement is micaceous or chloritie. 
——:— Pebbles. Felsite, quartzite, and granite, in the order named, 
are the most abundant constituents of the pebbles. Of twenty speci- 
mens examined fourteen contained felsite, thirteen quartzite, eight 
granite, and four quartz. Several other rocks of less frequent occur- 
rence were noted, including slate, contemporaneous sandstone, pegma- 
tite, and diorite. The felsite and quartzite are represented by many 
varieties. The granite is not like the bluish hornblende granite of 
Quincy, but, like that of the Boston Basin, is of the type found at 
Dedham and Randolph, with pink and green feldspars, biotite, and 
hornblende. The pegmatite and diorite were found in fine ledges 
by the roadside at Franklin VIII, R 21 (Plate 7). The former con- 
sists of albite and quartz and is the only specimen of its kind yet seen 
by the writer, in either the Norfolk Basin or the Boston Basin. 
Diorite is seldom found in the conglomerate. 
The size of the pebbles varies greatly. The coarse conglomerate 
along the south side of the Blue Hill Range contains pebbles two feet 
or more in diameter. At the ledges referred to in the previous para- 
graph many of the pebbles are six inches in diameter and some of them 
attain the size of one foot or more. At other localities the pebbles do 
not reach such dimensions but on the whole the rock may be called 
fairly coarse, the pebbles varying up to three or four inches in length. 
The shape is also variable. Of twenty-one specimens examined two 
contained angular pebbles, eighteen subangular, and five rounded. 
The same specimen may contain examples of each of these types. 
As in the case of the Boston Basin subangular types are the most 
abundant. 
A search for striations or markings on the pebbles that might be 
ascribed to glacial action was unsuccessful in this area as well as in 
the Boston Basin. Pressure-striated pebbles were found to be more 
numerous, however, than in the latter area. 
