218 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
schistosity are developed, but in such cases the strikes of the several 
planes are fairly uniform while the dips differ. The prevalent strikes 
of the shear planes are northeast or east-northeast and the dips, as a 
rule, are steep northerly. 
While it is true that there is comparatively little secondary develop- 
ment of the micaceous element, it often happens that rocks containing 
a large number of felsite pebbles of fine grained, homogeneous char- 
acter and greenish color, assume an unctuous appearance and feel. 
Such rocks have been described by Crosby in his earlier papers as 
pinite conglomerate. Where felsitic pebbles and finer material con- 
stitute the main elements of the conglomerate the rock is found to 
shade by almost imperceptible stages into felsite breccia and thence 
into compact felsite. Such gradations are to be observed at several 
localities, notably at Medford, Mattapan, Hyde Park, and South 
Natick. The impression left upon the observer after a study of such 
localities is that they represent cases where the matrix and pebbles 
or fragments consisting of one and the same kind of rock have been 
gradually welded by dynamic metamorphism into a compact mass 
entirely similar to the original rock. That such a type of metamor- 
phism may be found among rocks of this character has been shown 
by Dutton in his account of the volcanic conglomerates of the High 
Plateaus, previously quoted (page 116). 
The pebbles of the conglomerate in many localities are flattened, 
elongated, indented, and fractured, and often the matrix presents the 
appearance of flow structure. Sometimes open spaces have been 
left dt the ends of the pebbles by the movements induced by the 
shearing forces. Later infiltrations have filled the cavities thus pro- 
duced with quartz or calcite. The occurrence of such pebbles bears 
evidence of the presence of forces of deformation. ‘The localities 
where they occur, if carefully searched out and correlated in zones, 
would indicate the position of the axis of deformation. No extended 
search with this idea in mind has been made by the writer, but the 
observations at hand tend to show that the deformed pebbles occur 
in more or less well-defined bands, extending in an east-west or east- 
northeast-westsouthwest direction. For example, distorted pebbles 
occur at the outcrops near the railroad in Auburndale, West Newton 
and at the ledge on North Beacon Street in Brighton, where the s0- 
called slate pebbles occur. Again, they are found in the ledges north 
of the railroad at Newton Center, in the vicinity of Chestnut Hill 
Reservoir and at the great quarry on Tremont Street in Roxbury- 
a eee 
