328 Reports and Proceedings — Geological Societ// of London. 



The following communications were read : — 



1. " On some Quartzite Dykes in Mountain Limestone near 

 Suelston (Derbyshire)." By Henry Howe Arnold-Bemrose, Esq., 

 M.A., F.G.S. 



At Snelston, 3^ miles south-west by south of Ashbourne, there 

 is an inlier of Mountain Limestone surrounded by Keuper Marl. 

 It is roughly elliptical in shape, the major axis extending for 

 a distance of about half a mile north-north-east and south-south-west. 



The limestone is generally massive, with a few chert nodules in 

 the upper parts ; the rock in many places has a broken appearance, 

 and it contains small hollow spaces ; and large portions of the lime- 

 stone have been partly or completely dolomitized. The floor and 

 faces of the quarry are traversed by vertical veins or dykes of 

 calcite, fluor-spar, barytes, calcareous sandstone, and quartzite. 



The quartzite of these ' dykes ' is described microscopically. It 

 consists of angular detritus, quartz-grains with enclosures, a few 

 small grains of felspar, and a few shreds of mica. The grains are 

 cemented by silica, and sometimes by calcite. The rock in contact 

 with the dykes sometimes contains quartz in isolated bipyramidal 

 crystals and granular aggregates. The silica is present in the lime- 

 stone in two forms, which have had an entirely different origin. 



Reference is given to examples of sandstone dykes hitherto 

 described, and then the origin of the quartzite dykes at this locality 

 is discussed. An important bed of sandstone was found by sinking 

 for a well at Marston Common Farm ; and the same bed is found 

 also in a quarry about 800 feet south of the farm. The microscopic 

 aspect of the rock is precisely similar to that of the dykes. It is 

 at a period later than the Keuper that the silica which cemented 

 the sandstone of the dykes and of the Common Farm appears to 

 have been introduced. 



2. " Phenomena bearing upon the Age of the Lake of Geneva." 

 By C. S. Du Riche Preller, M.A., Ph.D.. A.M.I.C.E., M.LE.E., 

 F.R.S.E., F.G.S. 



Following up his investigations concerning the age of the 

 principal Alpine lake-basins, the author has, during a recent 

 prolonged stay on the Lake of Geneva, examined the low-level 

 gravel-beds and other alluvia to the Rhone Valley, from Geneva 

 to the Jura bar near Fort de I'Ecluse, as well as the high-level 

 gravel-beds of La Cote above Rolle and of the Jorat district above 

 Lausanne, and, further, the rock-formations on both sides of the 

 lake, in view of evidence of flexures as the primary cause of the 

 formation of the present deep lake-basin. 



After describing the phenomena around the Lake of Geneva, and 

 comparing them with those around the Lake of Zurich, he is led 

 to the following conclusions : — 



(1) The low-level gravel-beds of the Rhone Valley near Geneva 

 are, like the deep-level gravel-beds of the Limmat Valley 

 near Zurich, fluviatile deposits of the second interglacial 

 period, and were formed before the present deep lake-basin 

 came into existence. 



