DISTHIBUTION UNITED STATES 617 



Lake Superior region is regarded as adequate evidence that the ellipsoidal 

 structure is largely of subaqueous origin. 



"It should not be assumed, however, that all the ellipsoidal basalts of the 

 Lake Superior region are necessarily subaqueous. The region is a large one, 

 the conditions are varied, the ellipsoidal structures are locally associated with 

 structures ordinarily regarded as of subaerial origin, ellipsoidal structure is 

 known elsewhere to develop subaerially, hence it is rather likely that a part 

 of the structures in the Lake Superior region are of subaerial origin. . . . 

 Qualitatively the evidence favors the subaqueous origin of the major part of 

 the ellipsoidal basalts." 



Idaho. — Structures found in some of the basalts of the Snake Eiver 

 plains in Idaho seem to be comparable in many respects with pillow 

 lavas, although whether any of these are actually identical with pillow 

 structure, and if so to what extent they have been developed, must await 

 more detailed studies for determination. KusselF^ described a series of 

 seven parasitic cones that were formed on a bare corrugated lava stream 

 near its source. They are steep-sided and regular and range from 15 to 

 60 feet in height. They are composed principally of ^^rough ball-like 

 masses of highly vesicular or scoriaceous lava" 8 to 14 inches in diameter 

 and plastic enough to adhere to one another. 



Eussell found that two sheets near Hagerman showed contrasts be- 

 tween the upper and lower portions, the upper and central parts of the 

 flows being compact or moderately vesicular, while the lower is open in 

 texture and composed of irregular fragments, some extremely rough on 

 the surface and open and cellular within, like "a mass of irregular twigs 

 of glassy lava compressed into a moderately compact mass." There were 

 also many rounded masses "resembling a pillow folded on itself," the 

 lower surfaces of which are flat or concave from being molded on those 

 beneath. "These folded masses clearly indicate a rolling up of the still 

 plastic lava." There were also "oval or nearly spherical boulder-like 

 masses from a few inches to 2 or more feet in diameter, which are ex- 

 cessively hard and compact." The inner portions of these consist of 

 stony basalt with kernels of olivine and the surfaces are coated with 

 black glass 1 to 2 inches thick. Both the spheroidal and the pillow-like 

 masses are embedded in a coarse breccia of irregular glassy fragments. 



"All of the facts enumerated unite in indicating that the lava while yet 

 molten entered water and was quickly cooled, hardening to a glass, and that 

 the steam generated blew the still plastic material into shreds and excessively 

 irregular branching forms. As the upper surfaces of the lava sheets are not 



''^ I. C. Russell : Geology and water resources of the Snake River plains of Idaho. 

 Bull. U. S. Geol. Survey, No. 199, 1902, pp. 76-98, 113-117. 



