Vol. 57.] OF THE YELLOWSTONE AND GREAT BRITAIN. 219 



quartz-filled space itself becoming partly obliterated, owing to the 

 presence of feispathic outgrowths principally from the convex side 

 of the dividing walls. Passing towards the centre of the nodule we 

 find, at last, the crescent-shaped areas represented by irregular 

 grains of discoloured quartz, spread like cirrus-clouds through the 

 brown nodule. 



A perfect network of feispathic rods is seen also in many Boulay- 

 Bay slides occupying the end of a crescent-shaped space, where this 

 passes into the surrounding rock ; while adjacent parts of the 

 nodule appear to consist of feispathic fibres embedded in quartz. 

 In the same way a haphazard section of a spherulite may show 

 branching fibres of felspar projecting outward from a centre. 



The feispathic outgrowths common enough in the nodular rocks 

 of Eoulay Bay are exceptionally well seen in the very similar rock 

 from near Wrockwardine. In one instance the crescent-shaped 

 rings are formed about a central amygdaloid. These rings, now 

 consisting of infiltrated silica, are about '035 inch across, and each 

 is formed by two or more elliptical arcs joined together. From 

 the convex side of these arcs spring rod-like growths of felspar, 

 visible even to the naked eye, in a thin section. They stretch 

 occasionally half-way across the quartz-filled space, and, in some 

 instances at least, appear to have influenced the deposition of the 

 infiltrated silica, for the mammillated layers can be seen to bend 

 over the projecting fibres. (See fig. 2, II, p. 214, and fig. 3, p. 220.) 



ISTot uncommonly at Boulay Bay we find evidence to show that 

 porphyritic crystals of fairly large size, comparable with the fayalite 

 of the Yellowstone lithophysae, were present in this much older 

 rock. They occur in a rather puzzling series of specimens from 

 Boulay Bay and the Tete des Hougues, which possess a marked flow- 

 structure, and appear to have been very vesicular. 



A thin section of one rock shows that the greater part is 

 spherulitic, and possesses a mottled appearance due to the separation 

 of the particles during crystallization into a rather opaque greenish- 

 yellow substance and a more translucent yellowish-brown one : both 

 being fibrous. Many spherulites in the immediate neighbourhood 

 exhibit precisely the same structure. In this rock lie innumerable 

 amygdaloids, usually oval, and in some slides joined together so as 

 to produce a rather vermicular appearance. The original vesicles, 

 now filled wholly or partly with quartz, are sometimes lined by an 

 irregular spherulitic growth sta-ined with iron, containing specks of 

 opacite, and slightly coarser than the opaque material surrounding 

 it. Frequently we cannot define accurately the internal limit of 

 this spherulitic ring, since the earliest layer of infiltrated quartz 

 is usually discoloured. As in the hollow spherulites from Obsidian 

 Cliff, the interior of the vesicle must have been exceedingly rough 

 and uneven, and was probably provided with projecting spurs 

 penetrating the cavity. A thin section, passing near the edge of 

 such a spherulite after it had been filled up, would present a very 

 confused arrangement between the infiltrated and the original 

 materials. Occasionally a distinct lithophysal structure is found on 

 a small scale. 



q2 



