MANSFIELD: ROXBURY CONGLOMERATE. 171 
Basin rocks, for pegmatite is a frequent associate of the muscovite 
granite in the highlands northwest of the Boston Basin. 
Slate. Slate pebbles occur more commonly in the conglomerates 
of the Boston and Norfolk Basins than in the Narragansett Basin. 
The Lower and Middle Cambrian slates at Braintree and Weymouth 
indicate a local source for these materials. 
Sandstone, Grit, and Melaphyr. Occasional pebbles of greenish 
gray sandstone and grit, entirely similar to members of the conglomer- 
ate series, indicate that contemporaneous erosion accompanied the 
deposition of these rocks. Similar evidence is afforded by the occur- 
rence of melaphyr pebbles in certain parts of the Roxbury Conglom- 
erate. 
Summary of Sources. (1) The abundance of felsite, granite, and 
quartzite pebbles in the conglomerate, together with the occurrence 
of exposures of these rocks in proximity to the several basins, indicate 
that the material is locally derived and has not been transported 
great distances. 
(2) The generally local nature of the deposits militates against the 
idea that the fossiliferous quartzite pebbles came from Great Belle 
Island, while the increasing frequency of these pebbles toward the south 
and east indicates a probable source in a land mass, now no longer 
extant, in that direction. 
(3) The muscovitic material of the upper members of the Narragan- 
Sett series was probably derived from areas of rocks of that type now 
exposed in the highlands northwest of the Boston Basin. 
(4) The materials of the conglomerates were therefore probably 
Supplied from sources both north and south of the present areas and 
not from any single direction. 
(5) The deposition of the sediments was accompanied by contem- 
poraneous erosion. 
(6) The quartzite of the Harvard Conglomerate is unlike that of 
neighboring regions and resembles an aeolian deposit. 
GENERAL SUMMARY.— (1) The sediments of the three basins show 
great similarity in character, composition, and color, but red colors are 
less intense in the Boston Basin than in the others. 
(2) In all three basins the sediments are not very well assorted. 
Materials of many sizes and shapes are found in the same deposit. 
(3) The pebbles cannot generally be described as well rounded, but 
rather as subangular to rounded. The grains composing the grits and 
Sandstones and the matrices of the conglomerates are usually more 
angular. 
