■92 Correqjondence — Mr. J. R. Dahyns. 



vast masses of igneous rock previously cooled and denuded. The 

 author has identified and visited the sections, and finds in one that, 

 although there is a step-like junction between the sediments and 

 the igneous rocks, it is the result of intrusion of diabase, and not 

 of the deposition of sediment. The sediment, which is fossiliferous, 

 is converted into an intensely hard whitish marble, and the associated 

 shale bauds into chert. The diabase, which is ordinarily an ophitic 

 rock, acquires at the junction a finely crystalline-granular structure. 

 Jukes's second section also gives undoubted evidence of intrusion. 



Geological Society of London : Medals and Awards, 



January 10th, 1900. 

 The Council of this Society has decided to give the Medals and 

 Awards on February 16th, 1900, as follows: — 

 The " Wollaston Gold Medal " to the American Geologist, Professor 



G. K. Gilbert, For. Memb, Geol. Soc, of the United StEttes 



Geological Survey, Washington, D.C., U.S.A. 

 The " Murchison Medal" to Baron Adolf Erik Nordenskiold, Ph.D., 



For. Memb. Geol. Soc, of Stockholm. 

 The "Lyell Medal" to Mr. J. E. Marr, M.A., F.E.S., F.G.S., of 



Cambridge (for many years Secretary of the Geological 



Society of London), Fellow of St. John's College, Cambridge. 

 The Wollaston Fund to Mr. George Thurland Prior, M.A., F.G.S., 



of the Mineralogical Department, British Museum (Natural 



History), S.W. 

 The Murchison Fund to Mr. A. Vaughan Jennings, F.L.S., F.G.S., 



Assoc. R.S.M., etc. 

 The Lyell Fund to Miss G. L. Elles, Woodwardian Museum, 



Cambridge. 

 The Barlow-Jameson Fund (1) to Mr. G. C. Crick, Assoc. E.S.M., 



F.G.S., Geological Department, British Museum (Natural 



History), Cromwell Road, S.W. ; (2) to Professor T. T. 



Groom, M.A., D.Sc. Lond., F.G.S., of the College, Reading. 



COZE^ZRES^^OHiTIDlElsrGJE. 



THE COLOUR OF GLASLYN AND OF LLYN LLYDAW. 

 Sir, — Glaslyn and Llydaw are the names of the two chief 

 Snowdonian lakes. Glaslyn has from time immemorial been noted, 

 as its name implies, for the greenish colour of its water; but until 

 this year, 1899, there has not been anything peculiar about the 

 colour of Llydaw. Last summer, however, for the first time within at 

 least the last fifty j-ears, the water of Llydaw has become as green as 

 that of Glaslyn. The cause of this change of colour is not far to seek, 

 for last spring the company that works the Suowdon copper-mine 

 commenced crushing and washing their ore on the bank of Llydaw, 

 jso that a large quantity of greenish debris is daily carried into 

 the lake, whose water has thus become turbid and greenish in 

 icolour. The rock excavated along the copper veins is of a greenish 

 liue, as anyone may see by looking at the tips at the mouths of the 



