MANSFIELD: ROXBURY CONGLOMERATE. 215 
characteristics does not in itself constitute a valid argument against 
the possible occurrence of such rocks. It may very well be conceived 
that large masses of lava have risen through sedimentary rocks near 
to the surface without actually breaking through, or that such masses 
may even have broken through the surface at some stratigraphically 
higher level, so that both the higher sediments and the outpoured lavas 
were eroded away. In either case the lava close to the surface would 
partake largely of the character of rocks that were actually effusive. 
In the case of the Brighton rocks it does not seem to the writer that 
the irregularities of contact can be explained as the result of sedimen- 
tation on the irregular surface of the lava; for in specimens of sand- 
Stone and melaphyr contacts, examined on the polished surface with 
a microscope, the layers of sandstone appeared bent and broken as 
the contact was approached, and in the immediate vicinity of the 
latter the layers became fused to.a homogeneous, silicified mass, 
ighly ferruginous, in which the layer structure could no longer be 
distinguished. Even supposing the silicification and discoloration 
to be due to the causes suggested by Crosby it would seem hardly 
Possible that the layers would be bent and broken at the contact, 
were the latter sedimentary. Some of the larger features of the con- 
tact are shown in Plate 4, which gives views of the east and south 
Sides of the ledge a few hundred yards northeast of the sharp bend in 
Commonwealth Avenue (Boston V, I 28). On the eastern side (A) 
the irregular vertical contact is seen cutting across the bedding of the 
Sandstone. On the south (B) an irregular strip of sandstone appears 
Included in the melaphyr. Figure 7 (A) represents more minute 
features observed in a ledge back of the convent (Boston V, G H 27). 
In the region of Brookline and Newton the evidence of intrusive 
Contact is equally clear in many localities. The same characteristics 
of irregular igneous contact with the baked and discolored zone are 
to be seen at a number of localities, notably at a large flat ledge a few 
tods southeast of the city quarry at Thompsonville. There melaphyr 
and sandstone are exposed in the intimate relation shown in Figure 
7 (B). Evidence of the intrusive nature of the melaphyr is often 
Obtained at some distance from the actual exposures of that rock. 
or example, on the south side of Walnut Hill, at several points 
among the fine conglomerate exposures in that vicinity, the matrix of 
the conglomerate is impregnated with a dark red igneous rock which 
envelops the pebbles and mingles with the finer fragments. It is 
Usually local and irregular in occurrence and has often become schis- 
tose with the shearing of the rock. Similar impregnations have been 
