100 ANNALS NEW YORK ACADEMY OF SCIENCES 



several feet in thickness, and in many other places the lava was rendered 

 extremely vesicular. The upper portions were not so thoroughly impreg- 

 nated with steam but were, nevertheless, quickly cooled and became 

 viscous. The jets and tongues of fused material seem to have assumed 

 the consistency of a thick syrup and instead of spreading laterally they 

 solidified in smoothly rounded bowlder-like masses, having considerable 

 similarity to the "pahoehoe" of Hawaiian flows which were first reported 

 by J. D. Dana^- and more fully described and illustrated by C. E. Dut- 

 ton.^^ The pasty character of the fluid and its sluggish movements are 

 well attested in the billowy forms presented when quarrying operations 

 have attacked bodies of trap of this character (Plate X). The rounded 

 forms are sometimes built up to a thickness of 60—70 feet. The interior 



Fig. 1. Microlitic growth of phenocrysts iu vitrophyric crusts. X 35. Slide 132. 



of the bowlders cooled with sufficient slowness to permit the basalt to 

 crystallize with normal texture, but each is sheathed with a crust of glass 

 (tachylite) varying from an inch to several inches in thickness, having 

 often a laminated structure.^* Where unaltered, the color is usually dark 

 olive-green or brown, and the appearance is decidedly vitreous. Under 

 the microscope the rock is found to consist of a paste of glass, in which 

 are set a few phenocrysts of labradorite and diopside, commonly sur- 

 rounded by aggregates of hairlike microlites, which mark the continua- 

 tion of growth of the phenocrysts during the initial stages of chilling 

 before cr}^stal-growth was wholly blocked by increasing viscosity (fig. 1). 



13 U. S. Exploring Expedition — Geology, p. 162. 

 13 4th Ann. Kept. U. S. Geol. Surv., pp. 96 and 98, 1882-1883. 



" B. K. Emersox : "Liabase Pitchstone and Mud Enclosures of the Triassic Trap of 

 New England," Bull. Geol. Soc. Am., vol. 8, pp. 59-80, 1897. 



