MANSFIELD: ROXBURY CONGLOMERATE. 95 
bury, where it has long been known as the “Roxbury Pudding-Stone.” 
Since this rock is, on the whole, one of the most important and 
characteristic members of the series, its name has been extended 
to include the entire formation. 
Accompanying the Roxbury Conglomerate are contemporaneous 
intrusions or outflows of basic lava that are intimately related to the 
sedimentary series. These constitute a problem by themselves and 
will be discussed in this paper only in a general way. 
AREAL BOUNDARIES.— Maps. The earliest map of the Roxbury 
Conglomerate that has come to the notice of the writer was pub- 
lished in 1818 by J. F. and S. L. Dana in connection with their 
Outlines of the mineralogy and geology of Boston and its vicinity. 
On this map the conglomerate, designated as graywacke, appears as 
a belt about two miles wide and eight or ten miles long, extending in 
an east-southeast direction. In President Hitcheock’s map, published 
in 1833 (E. Hitchcock, a), the conglomerate, still called graywacke, 
is represented as a somewhat oval area with a southwest prolongation 
through South Natick. The only map that gives the boundaries of 
the region as a whole with any detail is Crosby’s geological map of 
eastern Massachusetts, published in 1877. In later years the same 
writer has published more detailed maps on a larger scale of the 
southwestern parts of the area, Nantasket and Cohasset (1893) and 
Hingham (1894). The southwestern extension of the conglomerate 
has been mapped in outline by Tilton (1895). These later studies, 
together with the work of Burr and the unpublished data collected 
by a number of students of advanced geology at Harvard University 
show that some modifications of Crosby’s earlier map are necessary 
but that in the main its outlines are substantially correct. In the 
preparation of the map accompanying the present paper (Plate 7) all 
these sources of information have been consulted, together with further 
papers by Professor Crosby and others and the field notes of the writer. 
The Northern Boundary. The true northern boundary of the 
Roxbury Conglomerate series is enveloped in some obscurity on account 
of the uncertainty regarding the age and stratigraphic position of the 
Cambridge and Somerville slates. ‘The latter are regarded by Crosby 
and others as conformably overlying the conglomerate and forming 
part of the same series with it; but there is room for a difference of 
opinion and the matter cannot be regarded as definitely settled. On 
the supposition that the slates form the upper members of the con- 
glomerate series, the northern boundary of the area is marked by a 
