BASIC VOLCANICS OF HEMLOCK FORMATION, 119 



Various attempts have been made to explain this pecuhar structure. 

 Bonney, Dathe, and Raisin regard contraction as the force which produced 

 the rounded masses. 



Dathe and Dahner show by the presence of the concentrically arranged 

 amygdules that the ellipsoids were units, and were formed before solidi- 

 fication of the rock. This arrangement of the amygdules, as well as the 

 arrang'ement illustrated in fig. 8 on page 113, precludes at once the idea that 

 the structure owes its origin to the well-known weathering process which 

 by exfoliation produces spheroidal blocks. 



Rothpletz and Williams look upon the ellipsoids as due to mechanical 

 forces which ground down the angles and edges of a fractured lava flow, 

 the idea of both authors apparently being that the fractures were long sub- 

 sequent to the movement of the flow. 



Ells and Lawson mention the structure as concretionary. 



Winchell considers the cases described by him as agglomeratic 

 accumulations . 



Cole and Grregory see in the masses evidence of lavas rolling over 

 among themselves. In the later paper, published alone, Gregory definitely 

 states that the lava first contracted into spheroids, which then rolled over 

 one another. 



Ransome explained the Point Bonita occurrence as a basalt which 

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

 upon another to form the whole mass of the flow."^ In the description of 

 the basalts, he writes: "A certain amount of crushed and sheared material 

 fills the interstices between the spheroids and seems to be made up of com- 

 minuted fragments of the same rock. It is, however, too crumbling and 

 too full of secondary products for a satisfactory determination."^ In the 

 second occurrence, in a fourchite (augitite'f), the relations of the rocks are 

 such as to prove "conclusively that such structure can not be rigidly 

 restricted to surface flows, although it is still believed that lavas exhibiting 

 it must have been erupted under very nearly surface conditions."^ 



Teall agi-ees with Ransome in comparing the ellipsoidally parted 

 masses of basalt to pahoehoe lava. Teall concludes that such ellipsoidally 

 parted basalts are submarine flows. 



In a recent paper Smith has described from certain volcanics bodies 



• Op. oit.,p. 112. 'Op. cit., p 78. ^ Angel Island, op. cit., p. 202. 



