Vol. 60.] 



IGNEOUS ROCKS OF PONTESFORD HILL. 



4(51 



empty spaces subsequently tilled with quartz, we should get a very 

 close resemblance to some of the nodules that are found in the 

 ancient rhvolites of Pontesford and elsewhere. 1 



Without entering, for the present, into a further detailed account 

 of these pyroineridal structures, the general conclusions so far arrived 

 at may be thus briefly summarized. In many cases, though 

 certainly not in all, the nodule appears to have commenced as 

 a vesicle, often irregular in shape, and sometimes, possibly, with 



Fig. 2. 



Complex vesicle in artificial slag. (Natural size.) 



crescentiform spaces around the main cavity, and separated from it 

 by similarly-shaped portions of the glass. Such vesicles probably 

 occur, on a very small scale, in the matrix of the rhyolite, and 

 show little or no further change, beyond the infilling of the 

 cavities with quartz and other secondary minerals, the fracturing 

 and deformation of their walls by subsequent movements of the 



1 There is a striking similarity between this lithophyse in slag and many of 

 those in the rocks of Obsidian Cliff described by Prof. J. P. Iddings, 7th Ann. 

 Kep. U.S. Geol. Snrv. 1885-86 (1888) pp. 265 et seqq. It should be noted that 

 while this vesicle occurs at the surface of the slag, and was due solely to 

 the rapid distension and cooling of the slaggy magma, the lithophyses of 

 Obsidian Cliff, and of the ancient rhyolites of Pontesford, Boulay Bay, etc., are 

 in the body of the rock, and may have been produced, in some cases, by the 

 progressive crystallization in a 'hydrous patch,' as explained by Prof. Iddings 

 and Mr. Parkinson. 



