440 PEOP. &. A. J. COLE AlTD ME. G. W. BUTLEE ON THE 



structure of this kind has been figured in Mr. Iddings's admirable 

 pajDor on ' Obsidian Cliff, Yellowstone National Park.' ^ 



In some of the vesicles nearest to the surface of the lava, the 

 brown zone is absent, but small cones of clear or dusky crystalline 

 fiibres have arisen, their apices projecting into the cavity, while their 

 bases are sunk in the wall. Where the development of rod-like 

 crystallites, such as form these cones, has taken place with anything 

 like regularity, a fibrous fringe is seen all round the vesicle, the grow- 

 ing points of the fibres being directed inwards. But that a great 

 jpart of this fibrous zone, as also of the cones, originates in the glass, 

 spreading outiuards as well as inwards, is shown by the fact that 

 the lines of crystallites, arranged by flow, pass continuously from 

 the matrix, through the brown layer, into the dusky fibrous material 

 (PI. XII. figs. 2 & 3). 



In the smallest examples, the fibres from opposite sides of the 

 vesicle may meet in the centre, and a ' spherulite ' results, which, 

 but for the occurrence of intermediate stages, might be supposed to 

 have originated entirely in divergent outgrowth. Thus there are 

 spherulites in one of our sections that might have arisen in a normal 

 manner ; but doubt is thrown upon their character by the enormous 

 preponderance of lithophysal forms. 



In the larger examples, the irregularity and partial character of 

 the growth at the centre usually suggest that they have resulted from 

 the infilling of a vesicle. In the case of one or more of the conical 

 aggregates, the ' stalk ' grows out distinctly into the cavity, not by- 

 its fibres becoming sensibly parallel, but by the deposition of new 

 fibres, originating at the point of convergence of the older ones and 

 thence radiating outwards, often from more than one centre (PI. XII. 

 fig. 3). These additions, if continued, produce the spheroidal 

 aggregates visible to the naked eye, and often result in actual 

 bridging of the cavity (PI. XII. fig. 4). At points along the 

 ' bridge ' further spherulitic deposition may take place ; fibrous 

 crystals of felspar, giving clear low colours between crossed nicols, 

 may spring across to the adjacent walls (PL XII. fig. 5); and 

 finally even a vesicle 2 mm. in diameter may become completely 

 infilled by spherulitic matter radiating from various centres within 

 it. Prequently two vesicles have opened into one another, leaving 

 a partition of glass projecting from opposite sides into the common 

 cavity ; and this breached partition serves as the foundation for a 

 crystalline ' bridge ' across the cavity. At the Porgia Vecchia, 

 spherulitic deposition, in a host of instances, has gone on so far 

 as to obscure the lithophysal features. The prevailing type of 

 ' spherulite,' indeed, both in Lipari and Yulcano, shows in section 

 a dusky fibrous central area, which may possess concentric as well 

 as radial structure ; outside this is a thin, colourless, but crystalline 

 zone, followed finally by a brown cloudy zone, irregular and of very 

 variable width. Our studies lead us to the conclusion that this 

 type of ' spherulite' owes its characters to the dual mode of growth 



^ Seventh Annual Eeport U. S. Geol. Survey, pi. xviii. fig. 3, p. 277. 



