314 



PRINCIPLES OF STRATIGRAPHY 



dumped from a large number of enormous caldrons and allowed 

 partly to run out, some masses running together and some advanc- 

 ing over preceding masses. (Fig. 55.) (Duton-9; Dana-Sip; 

 Hitchcock-i6:i'(5'o.) These hummocks form a rolling surface, with 

 fine wrinklings which produce the satin aspect. The pillowy masses 

 have sometimes a glassy exterior half an inch or less thick, forming 

 tachylite, in which a variolitic texture may sometimes be found. 

 The rough or aa lava presents a striking contrast with the pahoehoe. 

 "It consists mainly of clinkers, sometimes detached, sometimes par- 

 tially agglutinated together, with a bristly array of sharp, jagged, 



Fig. 55. Pahoehoe lava of Manna Loa, Hawaiian Islands. (After Dutton.) 



angular fragments of a compact character projecting up through 

 them." (9- 95-) The breaking up of the lava occurs during the 

 flow. The masses are sometimes piled together in confused heaps 

 to a height of 25 to 40 feet above the general surface. The individ- 

 ual fragments vary in size from an inch to ten feet, or even much 

 more. In texture the lava is usually less vesiculated than the 

 pahoehoe, not scoreaceous, but cavernous exteriorly. Large pro- 

 jecting masses of jagged lava occur, some having been noted on 

 southern Hawaii, of slablike form and very compact — "twenty feet 

 or more long, eight feet high and three to ten inches thick, standing 

 vertically together, with a curving over at the top somewhat like 

 gigantic shavings." " (8:10.) Associated with aa lava are lava 

 balls or pseudobombs of concentric structure, sometimes wrongly 

 taken for bombs. "These lava balls are smoothish exteriorly, more 



