MANSFIELD: ROXBURY CONGLOMERATE. 218 
theory. Nevertheless, at certain places on the south side of the mel- 
aphyr the suggestion of intrusive contact is strong. Little tongues 
of melaphyr appear to cut across the bedding of the slaty layers and 
to mingle with the matrix of the conglomerate so as to enfold the peb- 
bles. Perhaps the first feature may be accounted for by the deposition 
of slaty material in the minute crevices of the rough surface of the 
lava, while the second may be the result of the infiltration of fine 
mud, derived from the erosion of the surface of the melaphyr, into 
; i ad Na 
ol \ 
Conglomerate 
Melaphyr 
—— — — — — 3 — + 
Fic. 6.— Conglomerate and melaphyr at Hough’s Neck. 
the matrix of the conglomerate. Perhaps, too, after the surface of the 
flow had cooled, cracks in the crust thus formed permitted the extru- 
sion of the still molten lava below into the rapidly accumulating, 
unconsolidated sediments above. It must be admitted, however, that 
this latter supposition is hardly tenable since the only evidence of 
‚such cracks in the melaphyr is found in the numerous dikes that 
occur in it, and these latter, with one or two exceptions, do not cut 
