MUSEUM OF COMPARATIVE ZOOLOGY. 77 



face of Beparation, the vesicular trap may be seen to extend downward 

 for a thickness of ten feet or more, becoming gradually denser below. 

 The upper mass, on the contrary, maintains its dense texture to the top 

 of the quarry, a height of sixty feet above the vesicular mass below it. 

 The only way in which these facts can be explained seems to be to re- 

 gard the two masses as separate lava flows ; the lower one showing its 

 upper vesicular surface, which, at the few points where it happens to be 

 stripped bare, resembles the form of ropy lava, such as is called " pa- 

 hoe-hoe " in the Sandwich Islands ; the upper mass revealing only the 

 dense under part of a later flow, from which the original vesicular upper 

 surface has here been worn away. This is entirely in accord with what 

 has been learned elsewhere in the Triassic formation ; for if the lavas of 

 this portion of the district are extrusive, it is the most natural thing in 

 the world that the sheets should be composed of successive lava floods. 

 The time between the outpouring of the two sheets here must have been 

 short, for they are not separated by any deposit of sandstone or shale, 

 nor does the upper surface of the lower flow manifest signs of wearing 

 away, as it might if it had been long exposed to the weather above water 

 level. It may be here mentioned that a sheet of trap exposed in the 

 railroad cut between Springfield and Westfield in Massachusetts bears 

 evidence of at least three successive flows, the thinnest being only a few 

 feet thick. 



Another feature of the trap quarry is found in the bands of fragmental 

 material that traverse it from one end to the other, trending N. 57° to 

 68° E. One of the clearest of these runs along the margin of the upper 

 platform and extends across the high face of the quarry. It is from 

 two to four feet thick, stands nearly vertical, with a slight hade to the 

 west, and consists of angular fragments of trap of all sizes contained in 

 a matrix of what looks like sandstone, although it bears no distinct 

 marks of stratification. The trap walls of the band are of the same me- 

 dium texture as the rest of the upper flow, and are sharply defined ; the 

 trap fragments are of similar texture, without change from their margin 

 into the centre. They vary in size from minute grains to great blocks, 

 three or four feet across. Slickensided surfaces abound, generally 

 parallel to the walls and sometimes extending into blocks of trap, which 

 are slightly dislocated thereon. In short, these bands are fault breccias, 

 the trap fragments coming from the adjoining rock, while the sandstone 

 has been washed down the fracture from the beds that once overlay the 

 trap, but which at this point are now worn off". As their bearing is the 

 same as that of the faults already found from topographic evidence, it is 



