462 PKOF. W. S. BOULTON ON THE [Nov. I904, 



mass, and, in some cases, a slight development of the brown fibrous 

 material. But, in the case of the larger cavities, the brown fibrous 

 growth has developed conspicuously, encroaching upon the cavity, 

 as well as the surrounding matrix, evidently in much the same way 

 as in the admirably-described cases of the much smaller lithophyses 

 of the obsidian of the Rocche Eosse, Lipari. 1 Thus, fibrous, 

 radiating, or mushroom-shaped masses can frequently be seen 

 penetrating the 'vesicle,' now filled with quartz, and spreading 

 across smaller cavities in the surrounding matrix. This fibrous 

 growth starts in general from the wall of the vesicle or cavity, but 

 it may develop from other lines or points. Thus felspar-phenocrysts, 

 which appear to be more numerous in the vicinity of the vesicles 

 than elsewhere, frequently form the centres for radiating growths, 

 which, by their coalescence, help to form the boundary-wall of a 

 nodule. Possibly, some of the vesicles, with their borders of brown 

 fibrous and often spherulitic matter, remained empty for a long 

 time, for the wall is often much fractured, angular fragments of it 

 occurring iu the cavity, and now surrounded by concentric layers 

 of quartz and brown dusty or fibrous felspathic or microfelsitic 

 matter, usually with a well-marked spherulitic structure. Thus 

 the fibrous growth probably represents a phase of 

 the early devitrification of the glass, while the quartz, 

 chalcedony, and brown spherulitic aggregates were 

 introduced subsequently. Indeed, some of this fibrous matter 

 may represent the original crystallization of the magma during 

 cooling, rather than the devitrification of solidified glass." At the 

 same time, it would seem that the formation of the fibrous material 

 is not confined to one stage in the process of devitrification, for, 

 as already remarked, it is found traversing old spherulites, now 

 occupied by secondary quartz. 



There seems to be no limit to the size of such nodules, 

 for the fibrous material may successively surround smaller 

 individuals, producing composite nodules, of which the smaller 

 constituents may be of true lithophysal origin, their amygdaloids or 

 filled-up vesicles having a definite relation to their boundary-walls : 

 while the outer enveloping walls have no such related amygdaloids, 

 but, instead, smaller nodules which have played the part of vesicles 

 or phenocrysts in inducing devitrification in the form of a fibrous 

 layer. 



Nevertheless, it is clear, from a study of the Pontes ford nodules, 

 that some are quite solid to the core, without any quartz-amyg- 

 daloid, and with a more or less irregular, radial-fibrous structure. 

 These may be looked upon as imperfect spherulites or * skeleton- 

 spherulites,' that probably commenced to develop from the centre 

 outward, as in the ordinary type of small spherulite. Further, 

 it would be rash to deny that in some cases the centres of these 



' G. A. J. Cole & G. W. Butler ' On the Lithophyses in the Obsidian of 

 the Rocche Rosse, Lipari ' Quart. Journ. Geol. Soc. vol. xlviii (1892) p. 438. 



2 See J. Parkinson ' Some Igneous Rocks in North Pembrokeshire ' Quart. 

 Journ. Geol. Soc. vol. liii (1897) pp. 469-71. 



