618 J. V. LEWIS — OHIGIN OF PILLOW LAVAS 



glassy and do not show the other characteristics referred to, it appears that 

 the water which the still hot lava entered was shallow." 



In the Snake Eiver Canyon, 1 mile above Salmon River, a brecciated 

 and torn lava contains masses of stratified sand and clay np to 5 or 6 

 feet in diameter. In other places streams 30 to 40 feet thick rest on 

 thinly laminated white sandy clay, evidently lake bottom, without dis- 

 turbing it. The bottom is glassy and contains a few masses of the clay, 

 however, and is vesicular from the steam generated. 



California. — Ransome^^ first described the pillow structure in the erup- 

 tive rocks of Point Bonita, near San Francisco, California (plate 18, 

 figure 1), which include a glassy and in part porphyritic spheroidal ba- 

 salt composed of rounded, flattened, bale-like, pillow-like, and variously 

 twisted forms. Many of the masses are elongated and bolster-like, with 

 their longest axes arranged roughly parallel. Dark green amygdules are 

 numerous, increasing in size and number toward the center, and these 

 spongy central portions often weather out, leaving empty shells. Dimen- 

 sions range up to 3 and even 5 feet in diameter, and the masses are 

 molded on each other with crushed and sheared material between, so that 

 no empty spaces remain. 



The author considers it "evident that we are dealing with a structure 

 taken on by the lava at the time of its original fluidity and movement. 

 It is essentially a flow phenomenon." It is compared with ropy pahoehoe, 

 but is thought to have had greater viscosity and more sluggish movement, 

 causing the rope-like forms to thicken up and shorten. 



"In brief then it is supposed that the spheroidal basalt of Point Bonita 

 flowed as a viscous pahoehoe, one sluggish outwelling of lava being piled 

 upon another to form the whole mass of the flow." 



Winchell,^^ commenting editorially on Eansome^s description of the 

 Point Bonita rocks, contended that the spheroidal basalt is of pyroclastic 

 origin and not essentially different from the overlying bed of tufl. 



Small igneous masses with spheroidal structure on Angel Island were 

 regarded by Eansome^? as apophyses from the same magma that formed 

 the neighboring sill of f ourchite. Hence it was thought that the pillow 

 structure is not rigidly restricted to surface flows, but these masses ^^must 

 have been erupted under very nearly surface conditions." The rock con- 



''^ F, L. Ransome : The eruptive rocks of Point Bonita. Bull. Dept. of Geol,, Univ. of 

 Cal., vol. i, 1893, pp. 71-113. 



'» N. H. WincHell : Am. Geol., vol. xiv, 1894, pp. 321-326. 



80 F. L. Ransome : The geology of Angel Island. Bull. Dept. of Geol., Univ. of Cal., 

 vol. i, 1894, pp. 193-234. 



