Geological Society of London. 279 



The author was not prepared to correlate this Donegal series with 

 any American group ; but the lithological affinities were rather with 

 the Taconian than with the Huronian. 



The Kilmacrenan series, in which the granite is intrusive, was 

 described as crystalline, and older than the Lough-Foyle group. It 

 was mainly made up of micaceous, quartzose, hornblendic, and 

 hydro-magnesian schists, quartzites, and crystalline limestones. There 

 were no indications in these rocks of a metamorphism progressive in 

 the direction of the granite. This series was lithologically similar 

 to the Montalban system. 



Fifty-five microscopic slides of Donegal and Leinster rocks had 

 been examined by Prof. Bonney, whose observations confirmed 

 those of the author both as regards the nature and relations of the 

 granite and the general characters and state of crystallization of the 

 two schistose groups. 



2. " On Hollow Spherulites and their Occurrence in Ancient 

 British Lavas." By Grenville A. J. Cole, Esq., F.G.S. 



Many of the felstones of North Wales have been shown to be 

 altered lava-flows of an originally glassy type. In several localities, 

 as in the Pass of Llanberis, at the foot of the Glyder-fawr, these 

 rocks contain numerous nodular bodies, from i-q- inch to some inches 

 in diameter. The smaller varieties have the appearance of spheru- 

 lites, but the larger are ver^' often hollow, their cavities being partly 

 filled with minei'als deposited by infiltration. The lavas thus 

 receive a scoriaceous character, and have been described as slaggy 

 and vesicular. Similar structures, such as the " Lithophysen " of 

 von Eichthofen, occur in rocks of much later date, and the theory 

 advanced by Szabo, that their cavities have been formed by the 

 weathering-out of the centres of spherulites, has been pretty gene- 

 rally accepted. A consideration of hollow spherulites of very dif- 

 ferent sizes from Iceland, Lipari, and the Yellowstone area, tends 

 strongly to support this view, those portions that show radial struc- 

 ture being most easily attacked by the agents of decomposition. 

 The merely concentric coats of the spherulite, on the other hand, 

 consisting mainly of globulitic particles and glass, remain but little 

 altered, and a series of hollow shells may arise one within the other, 

 by the complete removal of the intervening radial matter. The 

 frequent association of perlitic structure and hollow spherulites in 

 the same rock may be due to the number of channels provided in 

 such cases for the passage of water or acid vapours. 



Structures resembling the " Lithophysen " of Hungary occur in 

 the altered rhyolites of the Wrekin, the cavities being filled with 

 quartz ; and the hollow nodules of the Silurian felsites in the Pass 

 of Llanberis prove, on microscopic examination, to have been origin- 

 ally spherulites. Many of these nodules show marked radial struc- 

 ture in their central areas ; and every gradation exists between tlie 

 solid varieties and those which have been hollowed out or replaced 

 by products of infiltration. The completion of such a process of 

 alteration raight cause a rock not originally vesicular to be regarded 

 as an ordinary amygdaloid. 



