DISTRIBUTION UNITED STATES 629 



trap also occurs beneath the "breccia/' "so that the latter must have been 

 formed in the midst of the sheet itself/' ^°^ by what process and through 

 what connection with the underlying wet sediment the author did not 

 undertake to explain. 



I have elsewhere presented evidence tending to show that the basalt 

 flows did not well out in a single vast eruption for each sheet, but that in 

 every case the great thickness was built up by a succession of smaller 

 flows or pulsations which spread themselves one on another. ^^^ Here and 

 there some of these subordinate flows encountered local pools of water or 

 saturated deposits of mud on the undulating surface of the underlying 

 lava, and the result was a frothing up of a small portion of the fresh flow 

 at the contact and an intimate mixture of the mud with the vesicular 

 mass. At Paterson one of these was overflowed with clean, solid basalt 

 before the formation of pillow lava began, and throughout the great thick- 

 ness of this zeolite-studded spheroidal mass there is no recognizable trace 

 of the underlying mud. In fact the pillows, almost entirely free from 

 vesicles and amygdules, are in striking contrast with the extremely scori- 

 aceous mass with which the mud is mingled. 



On the other hand, the pillow lava of Second Mountain is highly vesic- 

 ular, but is equally free from admixture of mud, although in many places 

 it is developed in immediate contact with the underlying sediments, and 

 the structure lies very near the bottom wherever it has been observed. 



Cheonologioal Table of Pillov^ Lavas, Pahoehoe, and Aa 



The descriptions that have been referred to in the preceding pages, to- 

 gether with certain pertiiient observations on the formation of pahoehoe 

 and aa lavas, have been listed in chronological order in the following 

 table. Theories of origin, discussed or implied, are briefly indicated in 

 the last column. 



Pillow lavas are to be understood unless otherwise indicated. 



"IB. K. Emerson : Geology of Old Hampshire County, Massachusetts, Monograph 

 U. S. Geol. Survey, vol. xxix, 1898, p. 424. 



lo^Ann. Kept, of State Geologist of N. J. for 1907, p. 150. 



