416 Geological Society : — 



physical conditions unlike those which gave rise to the camptonites 

 and bostonites. 



In discussing the general laws of differentiation the author 

 points out that it must have taken place before crystallization to 

 any extent had occurred, because there is a marked difference in 

 mineralogical composition between the rocks occurring as bosses and 

 those occurring as dykes ; and, further, that it is dependent on the 

 laws which determine the sequence of crystal-building, in so far as 

 the compounds which, on given conditions, would first crystallize 

 are those which have diffused to the cooling margin, and so produced 

 a contact-stratum, of peculiar chemical composition, before any 

 crystallization had taken place. 



2. ' On the Sequence of Perlitic and Spherulitic Structures (a 

 Rejoinder to Criticism).' By Frank Rutley, Esq., E.G.S. 



This paper relates to the order in which the perlitic and spheru- 

 litic structures have been developed in a felsitic lava of Ordovician 

 age from Long Sleddale, Westmoreland. The author having de- 

 scribed this rock in a paper, published in the Quarterly Journal of 

 the Society in 1884, and the accuracy of the views then expressed 

 having been questioned, now endeavours to confirm his original 

 statements, adducing in support fresh observations made upon this 

 and other rocks of a similar kind. 



3. * Enclosures of Quartz in Lava of Stromboli, etc., and the 

 Changes in Composition produced by them/ By Prof. H. J. 

 Johnston-Lavis, M.D., E.G.S. 



The author describes the existence of enclosures of quartz in a 

 lava-stream at the Punta Petrazza on the east side of Stromboli, 

 and also in the rock of the neck of Strombolicchio. He describes the 

 effects of the rocks upon the enclosures, concluding that the quartz 

 has undergone fluxion but not fusion, and has supplied silica to the 

 containing lavas, thus causing an increase in the amount of pyroxene 

 and a diminution in the amount of magnetite in the portions of 

 those lavas that surround the inclusions and raising the percentage 

 of silica. He suggests that such a process at greater depths and 

 higher temperature may, under certain conditions, convert a basic 

 rock into a more acid one, so that possibly the andesite of Strombo- 

 licchio may have been of basaltic character at an earlier period of 

 its progress towards the surface. He offers the suggestion that 

 other rocks or minerals once associated with the quartz have been 

 assimilated by the magma. 



December 6th, 1893.— W. H. Hudleston, Esq., M.A., E.R.S., 

 President, in the Chair. 



The following communications were read : — 



1. < The Purbeck Beds of the Yale of Wardour.' By the Rev. W. 

 R. Andrews, M.A., F.G.S., and A. J. Jukes-Browne, Esq., B. A., E.G.S. 



The authors have obtained better evidence than previously 



