224 IklR. J. PARKINSON ON THE HOLLOW SPHEKULITES [May IQOI, 



corrosion is that brecciation-cracks widen in passing from matrix 

 to spherulite.^ 



The nodules of Boulay Bay are frequently traversed by large 

 numbers of brecciation-veins of different ages,^ formed as a rule in 

 radial directions, since these are the planes of easiest parting. In 

 some the edges are distinct and sharp, in others they are blurred. 

 On the whole, they are more numerous in the nodules than in the 

 matrix. The older brecciation-veins are frequently clouded with 

 brownish material, and disappear almost completely from view 

 between crossed nicols, owing to the resemblance of the infilling 

 material to that which constitutes the walls. As described in a 

 former communication, a characteristic of these nodules is to break 

 up, between crossed nicols, into a mosaic of interlocking irregular 

 grains : often these are in optical continuity with the vein-substance. 

 Indeed, in the older cracks we can nearly always find evidence to 

 show that the sides are closely knit together, while in the newer the 

 edges are sharp cut.^ 



The polygonal contraction-cracks around sundry spherulites from 

 Boulay Bay do not appear abnormally wide, and they have of 

 course been exposed to the action of any hot springs or fumaroles 

 that may have existed. There seems, then, to be evidence, not of a 

 widening by corrosion, but of a healing of the old fracture, and it 

 is difficult to see how corrosion could act when once the vein was 

 filled by so refractory a cement as silica. Were it half filled, and 

 the means of transport less effectually blocked, the case might be 

 different.^ 



XI. Geneeal Conclfsions. 



In the second part of this paper I have endeavoured to elucidate 

 some of the structures of the old lavas of Great Britain by a 

 comparison with those of the Yellowstone region. 



We have found in the rocks of Boulay Bay and Wrockwardine 

 that the nodules contain amygdaloids of crescentic shape, passing 

 into more or less completely circular rings, and we have compared 

 these directly with the lithophysao of the American obsidian. 

 Moreover, we have found that felspathic fibres project outwards 

 into a quartz-amygdaloid, that parts of some nodules show an 

 open network of similar fibres now embedded in secondary material, 

 and we have been able to draw a direct analogy with the porous 

 spherulites of the Yellowstone. Pinally, traces have been found of 

 an encrusting mineral curiously resembling tridymite. All this 

 militates strongly against an hypothesis of corrosion, so that we 

 need feel no hesitation in applying to the older spherulites of Great 

 Britain a conclusion which appears in accord with the facts observed 

 in the rocks of the National Park of the United States. 



In the case of the large amygdaloids, irregular, partly circu 



1 Quart. Journ. Geol. Soe. vol. xlii (1886) p. 185 & pi. ix, fig. 2. 



2 Ibid. vol. hv (1898) p. 115 & pi. vii, fig. 1. 

 ^ The brecciation-veins traversing the nodules of Pontesford Hill pr 



identical features. 



