BASIC VOLGANIGS OF HEMLOGK FORMATION. 123 



concentrated upon the periphery with few or only microscopical cavities in 

 the center ; (3) they are concentrated on one side of the ellipsoid, this side 

 representing apparently that side of the ellipsoid turned toward the upper 

 surface of the lava stream. The following explanation is offered for this 

 difference in occurrence. The distribution of cavities is determined by 

 three factors : The viscosity of the lava ; the difference in specific gravity 

 between the bubbles filling the cavities and the lava ; and the expansive 

 action of the gas. In the case of (1) the ellipsoids are considered to have 

 consisted of lava in a viscous condition through wliich the gas pores formed, 

 but in which, owing to the high degree of viscosity, they remained nearly 

 or quite in the positions in which they were formed. Here viscosity was 

 the determining factor. In case (2) the gas pores, influenced chiefly b-s" the 

 expansion of the gas, collected upon the periphery — just as, for instance, in 

 the steel ingot while the center is compact the outer surface is porous. 

 The lava in this case was probably less viscous than in the former. In the 

 last described condition of distribution (3), where the gas cavities are on 

 one side, which is the upper surface, the lava was still less viscous than in 

 the preceding cases. Here specific gravity was the controlling factor, and, 

 as a result of the specific gravity and the less viscoiis nature of the lava, 

 the gas bubbles rose and collected upon the ^^pper surface. 



The explanation of the ellipsoidal basalts which has been off"ered — viz, 

 that they are comparable with aa or block lava — seems to offer a ready 

 explanation for all of the observed characters. On the whole, the ellip- 

 soids owe their origin and certain peculiarities to the viscous nature of the 

 lava. They possess also characters which are due to contraction, others 

 which are due to original flowage, and still others which ai-e the result of 

 subsequent orogenic movements. 



In certain places we may find the ellipsoids only half formed — that is, 

 attached by one side to the main unbi'oken part of the lava flow, the other 

 side showing a rounded outline. This probably represents a place where 

 the aa grades into a pahoehoe or smooth-flowing form. Such an instance 

 is possibly that illustrated by Ransome.' 



Both Ransome and Teall compare the ellipsoidal basalts studied by 

 them with pahoehoe lava. The latter also suggests a submarine origin for 

 the basalts studied by him. It should be noted that pahoehoe lava in its 



' Point Bonita, op. cit., fig. 2, p. 77. 



