392 Geological Society. 
thus receive a scoriaceous character, and have been described as 
slagey and vesicular. Similar structures, such as the “ Lithophysen ” 
of Von Richthofen, 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 ori- 
ginally spherulites. Many of these nodules show marked radial 
structure in their central areas ; and every gradation exists between 
the solid varieties and those which have been hollowed out or re- 
placed by products of infiltration. The completion of such a process 
of alteration might cause a rock not originally vesicular to be re- 
garded as an ordinary amygdaloid. 
April 15.—Prof. T.G. Bonney, D.Sc., LL.D., F.R.S., President, 
in the Chair. 
The following communications were read :— 
1. “ A General Section of the Bagshot Strata from Aldershot to 
Wokingham.” By the Rev. A. Irving, B.Sc., B.A., F.G.S. 
The Author referred to earlier papers in the ‘ Geological Magazine,’ 
in which the green colouring-matter so common in the Middle and 
Lower Bagshot strata of the London Basin had been attributed to 
the presence of vegetable débris and the materials resulting from 
decomposition of vegetable matter. The marked difference in this 
respect between these strata and the higher members of the series 
furnishes a clue to the conditions under which they were respec- 
tively deposited, the former being delta- and lagoon-deposits, the 
latter the deposits of a marine estuary. This implies a transgressive 
overlap of the upper portions of the Bagshot series upon the London 
Clay ; and the present paper was devoted to a consideration of the 
statigraphical evidence of this overlap. 
Sections were described in detail at Aldershot, Farnborough, 
Yateley, Camberley, Wellington College and the neighbourhood, and 
from the last-named place to Wokingham. From these, a general 
section was constructed to exact scale, both as to thickness of strata 
and altitudes, showing a relation of the Bagshot formation to the 
London Clay, which was inconsistent with the generally received idea 
