124 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
Conglomerate.— At two points in the area covered I have had 
the opportunity to examine the conglomerate. The first is in the 
locality north of the North Common Hill quarries. Wadsworth (’82, 
p. 132) and Crosby (’80) have both spoken of the rock as composed 
of debris of the Quincy granite. The rock is variable in character. 
On the south it is a compact conglomerate of angular to partially 
rounded granite pebbles, up to several centimeters in diameter, in a 
gray subcrystalline ground mass. Weathered surface slabs show 
well-rounded pebbles. The rock is in general considerably sheared 
and is especially so on the north in the cut of the Old Colony R. R. 
where certain portions look almost like a slate. In the latter cut, 
north of Black’s Creek, the planes of the included calcite crystals 
show shearing notably. The pebbles here are of quite small size. 
From the field relations and from the rounded character of the 
pebbles seen in the hand specimen, we must conclude that the rock 
is a sedimentary conglomerate, yet in thin sections the ground mass 
cementing these pebbles is exceedingly like that of a volcanic tuff, 
as seen in PI. 2 , Fig. 7. It consists largely of rather large lath¬ 
shaped plagioclases and short stubby hornblende crystals with 
scattered grains of magnetite. Plagioclases of various sizes and 
irregular forms are packed into this ground mass. The feldspar is 
chiefly decayed, and does not show twinning. A handsome zonal 
structure is seen in some cases. Quartz is in small grains, scattered 
among the feldspar. Fractured grains of epidote occur through the 
sections. Many of the smaller pebbles at least are of fine-grained 
porphyry. 
Unless the rock is in reality a true volcanic which has in some 
way enveloped rounded pebbles, we have a case of a conglomerate 
surprisingly metamorphosed and recrystallized in the presence of 
the neighboring igneous rocks. It should, however, be observed that 
rounded pebbles show only in weathered blocks; in hand specimens 
of fresh material the imbedded pebbles seem to have a more angular 
section (PI. 4 , Fig. 13) . 
The conglomerate of Hough’s Neck, as Professor Crosby clearly 
demonstrates, is of two ages of deposition, one underlying, the other 
overlying, the interbedded sheet of melaphyr. The lower conglom¬ 
erate somewhat resembles the one above mentioned, but is not of 
nearly so fine a texture. Much crushing has evidently taken place, 
and the result in this section is unsatisfactory. The upper con¬ 
glomerate is a much coarser rock, often containing large included 
