1880.} A427 | [Davis. 
In the field north-west of the corner of Warren and Allston Sts., 
on a small rocky ledge, is a little opening known to some as the “ Briar 
Patch Quarry.” It exposes very clearly a dike twelve to fifteen feet 
wide, running about north through thin-bedded slates, of which the 
dip is locally somewhat disturbed. A similar dike cuts the ripple- 
marked sandstone quarry on Warren St. Without conclusive evidence 
on the subject, Ihave considered it probable that these dikes which 
come up from below, breaking across the slates, are feeders of the 
amygdaloid masses, which lie at small angles with the strata (v. the 
preceding paper by E.R. Benton). The sides of the dikes are 
to be especially noted. They are extremely irregular as if torn 
apart, and when contrasted with the melaphyr dikes.of this dis- 
trict which have comparatively straight sides, imply an eruption 
before the production of joint-planes in the enclosing slate. As 
joint-planes are common in horizontal rocks of considerable age, and 
may be said to occur always in tilted rocks, the evidence of these 
