Geological Society. 143 



plane-polarized light is incident, and the light emerging from 

 the interposed crystal is resolved into two streams of plane- 

 polarized light (by the analysing plate), of which one only is 

 transmitted to the eye. It is known that circularly or ellipti- 

 cally polarized light will, if incident on the crystal, form rings ; 

 but it has not been remarked as a general theorem, that rings 

 will be visible if the analysing plate be so constructed as to 

 resolve the light emerging from the crystal into any two kinds 

 of light, of which it suppresses one and transmits the other to 

 the eye. Now by means of Fresnel's rhomb, or (imperfectly) 

 by a film of mica, the analysing plate may be made to resolve 

 the emergent light into two circularly-polarized rays, one of 

 which it transmits to the eye, while the other is suppressed. 

 Supposing the light to be thus analysed and supposing the 

 incident light to be circularly polarized, theory gives this 

 result : the tint will depend only on the gain or loss of the 

 extraordinary on the ordinary ray : there will be no brushes : 

 the appearances will not alter as the crystal is turned about 

 the incident ray. These conclusions are completely supported 

 by experiments on uniaxal and biaxal crystals and unan- 

 nealed glass. Iceland spar, for instance, shows rings without 

 brushes : nitre, &c. exhibit the lemniscates in their whole ex- 

 tent without any interruption. 

 Cambridge, July 25, 1831. 



XIX. Proceedings of Learned Societies. 



GEOLOGICAL SOCIETY. 



June 6th.— A LETTER was read, from Joshua Trimmer, Esq. 

 ■L\. to the Rev. Dr. Buckland,V.P.G.S.*' On the diluvial 

 deposits of Caernarvonshire, between the Snowdon chain of hills and 

 the Menai strait, and on the discovery of marine shells in diluvial 

 sand and gravel on the summit of Moel Tryfane, near Caernarvon, 

 1000ft above the level of the sea." 



The object of this paper is to point out evidences of extensive dilu- 

 vial action in that part of Caernarvonshire which lies at and near the 

 N.W. base of the mountains of Snowdonia. This district is traversed 

 in a direction from N.E. to S.W., and nearly parallel to the mountain 

 chain, by two remarkable beds of roofing slate, well known by the 

 name of Penvhyn Slate, dipping usually to the S.E. at a considerable 

 angle, and extending along a series of hills of moderate elevation, 

 between the Snowdonian chain and the Menai strait. Great part of 

 the surface of these hills, and of the still lower ground between them 

 and the Menai, is so covered by accumulations of drifted gravel, sand 

 and clay, that the slate is seldom accessible, without first quarrying 



down 



