Correspondence. 5115 



eaten away so as to form the Vale of Winscombe? Professor 

 Jukes implies that the sea, during the Glacial submergence, did no 

 more to make valleys than a canal is made by the water it contains. 

 I think the contrary can be shown by three facts : 1. The rate at 

 which the sea wears back its cliffs in many parts of the Bristol 

 Channel may be fairly stated as, at least, a foot in a year. 2. The 

 accumulations of drift in the midland and other counties (which 

 exhibit no trace of being re-arranged Tertiary gravels, and which 

 are only a part of what must have been excavated and removed to 

 a distance,) would be sufficient to fill up many valleys, and obliterate 

 many escarpments. 3. The duration of the glacial submergence 

 may have been at least 50,000 years, probably much longer. 

 During this period the sea must have converted many -~-^ shaped 



vales into i i shaped plains, and may have eaten back many miles 



of the eastern side of the Severn valley so as to leave the great 

 Cotswold escarpment. 



D. Mackintosh. 



P.S. — T see that the number of the Quart. Journ. Geol. Soc. for 

 the present month (Nov.) contains several important articles in 

 favour of mariae denudation. 



KESEARCHES IN BRITISH MINERALOGY. 

 To the Editor of the Geological Magazine. 

 Sir, — Mr. David Forbes in a paper in the November number of 

 the London and Edinburgh Philosophical Maga%ine, under the title of 

 " Eesearches in British Mineralogy," gives the results of an analysis 

 made by himself of a silver-fahlerz from the Fox-dale silver-lead 

 mine, in the Isle of Man, and in the introductory paragraph he says, 

 " Although the cupriferous tetrahedrite (occasionally containing 

 traces of silver) has been found in small quantities at various locali- 

 ties in both England, Scotland, Ireland, and Wales, there is no 

 analysis of true silver-fahlerz or polytelite, or even occurrence of 

 the mineral itself recorded, as far as the author has been enabled to 

 ascertain." From this I infer that Mr. Forbes will be surprised to 

 learn that silver-fahlerz has already been found in quantity in this 

 country and mined for the silver it contains. For several years 

 past it has been raised and sold as a silver and copper ore at the 

 Silver- vein Mine, near Lostwithiel, Cornwall. Indeed this mine is, 

 and has been worked solely for the sUver-fahlerz, no other ore being 

 found in any useful quantity. The lode (for it is not found in 

 " pockets" only) runs about 43 degrees east of north and west of 

 south. Its width appears to have varied considerably, but at the 

 present time it is about four feet wide. It traverses the " Killas" 

 or clay-slate of the district and, so far, the ore has become richer in 

 silver as the depth increases. I know of no accurate analysis having 

 been made of this ore, so that Mr. Forbes would be doing further 

 good service to British Mineralogy if he would take such a work in 

 hand. From eight assays made by Messrs. Johnson and Johnson 

 and others, some years since, the average yield of silver was GSi 



