I 12 



University of California. 



[Vol. i. 



spheroidal parting subsequent to solidification, or for a rounding by 

 brecciation. 



It is when we turn to the forms of structure exhibited by the 

 modern basic lavas of the Hawaiian Islands, that abundant points of 

 analogy appear, and it becomes 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 florv phenomenon. As is well known, 

 the Hawaiian lava is erupted in two forms, — as aa and as pahoehoe. 

 The latter is described as being, at the time of its eruption, as "liquid 

 as water," * and, as may be seen in the illustrations of Dana f and 

 Dutton,J solidifies in smooth ropy forms, very suggestive of the 

 bale-like masses shown in PI. 7, Fig. 1. Now, if by any means the 

 viscosity of this pahoehoe could be slightly increased, its movement 

 would become more sluggish, and the rope-like forms would thicken 

 up, shorten, and approach more nearly the form shown in Fig. 2 ; or, 

 in other words, we might have a form of lava intermediate between 

 the pahoehoe and the aa. It is in this connection that the high per- 

 centage of titanium shown 'in the analyses becomes of importance, 

 and it seems quite possible that to its presence may be due the sup- 

 posed slightly greater viscosity required by the theory. 



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. 

 In portions of the area this structure remains substantially unmodi- 

 fied, as shown in PI. 7, Fig. 1 ; but elsewhere these bales, while still 

 plastic, were subjected to forces incident upon the forward move- 

 ment of the whole mass, and were rolled into more spherical forms, 

 or flattened into lenses. It is to be noted that this hypothesis har- 

 monizes with two facts observed in connection with the bales near the 

 lighthouse (PI. 7, Fig. 1), viz., that the searching action of waves 

 and weather reveals in them a more or less distinct differentiation 

 into an exterior shell and an inner core, and that the microscopic 

 flow structure is more conspicuous in the core than in the shell; for, 

 as the crust of each bale cooled, the central portion would still tend 



* See Dana, Characteristics of Volcanoes, p. 192. 

 t Loc. cit., Pis. VII and XI. 



+ Hawaiian Volcanoes, Fourth Ann. Report U. S. Geol. Surv., PI. XVIII. 



