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 puraice-like surface 

 of the lower sheet ; other grains are amygdaloidal, and contain small 

 ledges of some tricliuic 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 

 scoriae 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 tine-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 are 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 



