128 BULLETIN OF THE 
ward, and at four feet below the junction it is replaced by an earthy 
blue-green trap having abundant amygdules of chlorite and calcite, and 
to the eye appearing much fresher than the reddish trap. 
Numerous sections were cut from the red superficial portion of the 
lower sheet, and from its contact with the dense trap of the upper sheet, 
in order to detect any clastic material that might occur there. Very 
little was found, but immediately upon the upper surface of the lower 
sheet a thin layer was discovered consisting of -rudely stratified grains of 
clastic quartz and orthoclase, mixed with angular fragments of trap, 
like that of the red seam. Some of the trap grains are glassy, non- 
polarizing, and of a light green color, thickly sprinkled with minute 
dots of ferrite. They are probably fragments of the pumice-like surface 
of the lower sheet; other grains are amygdaloidal, and contain small 
ledges of some triclinic feldspar. The whole is cemented together by 
quartz and calcite. There is no marked tendency towards a stratified 
arrangement of the grains, such as characterizes deposition in water. 
The trap grains appear to have been the result of the comminution of 
scorize on the surface of the lower sheet during the ordinary progress of 
subaerial erosion, while the occasional grains of orthoclase or quartz 
may have been deposited by wind or stream action; and from this we 
have supposed that the thickness of the lower sheet was somewhat 
greater than the depth of the water into which it flowed. Hitchcock 
long ago noted that the reptilian tracks in the sandstones in Massa- 
chusetts occurred chiefly in the beds closely overlying the trap sheets, 
as if the depth of the Triassic estuary had been decreased for a time by 
the lava that had flowed into it. 
The lower trap of the quarry at ten feet below the red seam, where it 
is the least altered as far as the quarry exposes it, is fine-grained, of 
a dark greenish blue color, and of a uniform texture, containing abun- 
dant amygdaloidal cavities. Mineralogically it is composed of extremely 
altered porphyritic crystals of plagioclase in a ground mass of minute 
crystals of the same, which are in turn set in a matrix of the unindi- 
vidualized base. The base in places is a yellowish green glass, and in 
others is wholly devitrified. The augite that it undoubtedly contained 
originally has been entirely removed by alteration. Calcite and secon- 
dary quartz aré abundant, the former so plentiful that the rock effer- 
vesces readily, even with very dilute hydrochloric acid. Under the 
microscope, the rock appears profoundly decomposed ; its numerous 
amygdules being due to replacement, with the occasional exception of a 
well-outlined cavity, the result of gas expansion. Admitting the original 
