622 J. V. LEWIS ORIGIN OF PILLOW LAVAS 



diameter, the actual boundaries of which extend below the surface of the 

 flow. These are not products of erosion and, according to the author, 

 must be regarded as original structures. One flow is described as con- 

 sisting of rounded pseudobombs thickly and uniformly scattered through 

 the middle and upper parts of the sheet and wanting at the bottom. The 

 author offers the tentative explanation that "during the flow of the lava 

 layers and crusts were, by the unequal flowing and revolving motions, 

 broken up and the fragments rounded into the forms we now see." 



Emerson®^ has described the Triassic basalt sheet of Deerfield, Massa- 

 chusetts, a portion of which had formerly been regarded as an agglom- 

 erate, the masses being separated in part by stringers of sandstone like 

 that on which the sheet rests. 



"Many of the blocks have rows of these cavities [steam vesicles] around 

 their borders in whole or in part, and these cavities are tubular at times and 

 closely set at right angles to the fissure which separates the block from its 

 neighbor. . . . [This arrangement] shows that the slow expansion of the 

 steam was effective after the mass had cracked into great blocks." 



Some of the blocks higher up seem to have been invaded by later lava, 

 which, it was thought, sheathed the blocks and partially remelted them 

 into a spheroidal form. Other masses are separated by an admixture of 

 glass fragments and red sand. At the tower of Greenfield there is no 

 basal bed of normal trap ; 



"but the whole mass was cooled nearly to the crystallizing point when the 

 sand rose up into it at almost equal intervals, and the streams of sand and 

 glass breccia formed by the water rise in great streaks or 'schlieren,' anasto- 

 mose, and pass with fluidal structure around the great rounded blocks of the 

 normal trap. . . . The rounded bomb-like masses and the compact crystal- 

 line trap which are contained in this breccia grade superficially through 

 hyalopilitic trap into the green glass, and while compact at the center are 

 toward the surface full of radiating steam pores. . . . Among the blocks 

 are many long sheets and rounded masses connected by narrow necks, which 

 could not have been blown into the air and have fallen as common bombs." 



Typical pillow lavas are shown in- the illustrations, one of which I am 

 permitted to reproduce by courtesy of Professor Emerson (see plate 24). 



Connecticut. — ^^Emerson^^ compares the structures described in the pre- 

 ceding paragraphs with similar beds near Meriden, Connecticut, and 

 attributes both to a rapid submarine flow of lava, while steam from the 

 underlying water-soaked sediments broke the bottom crust and the mud 



89 B. K. Emerson : Diabase pitchstone and mud inclosures of the Triassic trap of New 

 England. Bull. Geol. Soc. Am., vol. 8, 1897. pp. 59-86. Also Geology of Old Hampshire 

 County. Massachusetts. Monograph U. S. Geol. Survey, vol. xxix, 1898, pp. 418-431. 



^ B. K. Emerson : Loc. cit. 



