Shaler. ] ie 2, [January 15, 
which gives some curious evidence as to the history of its movements. 
At this point many of the gas cavities have been in part filled in with 
a reddish rock resembling jasper, these reddish deposits have been evi- 
dently laid down from water by gravitation, as they fill only a portion 
of the space and are bounded above by distinct plane surfaces which 
are parallel in the various neighboring cavities.! These planes which 
bound the reddish filling of the blebs are now tilted at high angles 
to the horizon, and show that the mass has undergone extensive 
movements since its conversion into trap. 
The general character of this volcanic mass makes it seem probable 
that it was a true volcanic ejection, poured out towards theold land or 
ocean floor, and not an ejection stopping short of the surface, as is 
the case with most of our dykes. It is not likely, however, that the 
portion now left here represents the lava extruded from the voleanic 
vent; itis probable that we have only the material filling the fissure 
up which the volcanic matters travelled on their road to the surface. 
The interesting group of rocks considered in this paper occupies 
only a small space in-this region, its form is that of a strip some hun- 
dred feet wide, and extending from near Bigelow Street on the west, 
to near the junction of North Beacon and Chester Streets. It may 
extend somewhat further to the east and west, but if so it is masked 
by the coating of drift that covers it. 
At the pumping house quarry, and that near the junction of 
Chester Street and North Beacon Street the amygdaloid is seen to 
emerge from below, a series of thin bedded compact shales follow- 
ing the plane of their bedding. The exact contact of the two form- 
ations, however, is not seen at either place; at the former of these 
localities the contact of the amygdaloid with the lower lying thick 
bedded slates is seen in the wall of rock that borders the side- 
walk a few feet to the west of the pumping house. We there see 
that the slates are little changed by their contact with the amygda- 
loid, showing that the mass was not very much heated at the time 
of itsejection. The position of this amygdaloid in an apparent inter- 
bedded relation to the over and underlying rocks, would at first sight 
lead the observer to suspect that it was what is sometimes called a 
contemporaneous trap, i. €., a mass poured out on the sea floor and 
1 These infiltrations of jasper were first noticed by Mr. W. M. Davis, Jr., whose 
interesting observations on this and other structural peculiarities of these rocks 
are as yet unpublished. 
