624 J. V. LEWIS ORIGIN OF PILLOW LAVAS 



"pahoehoe or ropy rolling surfaces produced by the flowing of the viscous 

 lava . . . the rounded billowy forms of which are often covered with 

 dark glass one-half an inch to one inch thick. Often these ropy glassy sur- 

 faces are also vesicular or amygdaloidal. In some of the quarries such 

 rounded forms are superimposed to a depth of 50 to 75 feet and the cavernous 

 spaces between them have been partially filled with calcite, quartz, and 

 zeolites." ^ 



Fenner^^ also described the pillow structure at Paterson as pahoehoe, 

 and ascribed its origin to the flow of lava into shallow water with muddy 

 bottom. First a thin flow caused a violent agitation and mixing of the 

 lava and mud; successive spurts and tongues of fused material, chilled 

 suddenly by steam rising around them, built up the structure of boulder- 

 like formE. The original surface water was quickly driven off, but the 

 wet sediments continued to furnish steam, some of which entered the 

 lava and made it vesicular, but most of it worked its way around the 

 masses and quickly cooled the crusts to glass. 



"The jets and tongues of fused material seem to have assumed the con- 

 sistency 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. . . . The pasty character of the fluid and 

 its sluggish movements are weU attested in the billowy forms presented when 

 quarrying operations have attacked bodies of trap of this character. The 

 rounded forms are sometimes built up to a thickness of 60-70 feet. The in- 

 terior 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. . . . The crusts frequently present a shattered ap- 

 pearance due to the sudden chill which they experienced, and at times pockets 

 among the bowlders are fllled with considerable masses of breccia of this 

 nature." 



Pillow structure in Xew Jersey, some examples of which have been 

 described briefly in the preceding paragraphs, I have found typically 

 developed in the Triassic (Xewark) basalts of both First and Second 

 Watchung Mountains. In First Mountain the structure may be traced 

 in surface outcrops, quarries, and street and railway cuttings from the 

 corner of Hamburg Avenue and Jane Street, one-half mile northwest of 

 Passaic Falls, in Paterson, in a slightly curved belt near the middle of 

 the trap outcrop, in a direction about south 25° west to the trap quarry 



^ J. Volney Lewis : Petrography of the Newark igneous rocks of New Jersey. Ann. 

 Rept. State Geologist of N. J. for 1907, p. 152. 



^"^ C N. Fenner : Features indicative of physiographic conditions prevailing at the time 

 of the trap extrusions in New Jersey. Jour. Geol., vol. xvi, 1908, pp. 299-327 ; also The 

 Watchung hasalt and the paragenesis of its zeolites and other secondary minerals. Au- 

 nals N. Y. Acad. Sci., vol. xx, 1910, pp. 99, 100. 



