350 Mr. D. Forbes's Researches in 



At first sight it would seem that the resulting products in 

 these two instances are very different, although undoubtedly 

 both are derived from the alteration of the same rock-mass. 



This apparent discrepancy, however, is easily accounted for. 

 In the case of the first-mentioned dark-red ferruginous clay, the 

 alteration, having taken place at the surface, under the full oxi- 

 dating agency of the atmosphere, caused the whole, cr at least 

 the greater part of the iron present in the rock to be converted 

 into sesquioxide or red oxide of iron, and thus accounts for the 

 colour of the clay. 



In the depths of the coal-pit, however, cut off from access to 

 the air, the chemical changes would be very different. The 

 water percolating slowly through the substance of the rock, be- 

 sides causing it to become hydrated and disintegrated, would 

 carry off a portion off its lime, magnesia, and alkali, and proba- 

 bly introduce a little carbonic acid to combine with a portion of 

 the remainder of these bases ; yet at the same time the deoxi- 

 dating action of the carbonaceous matter in immediate contact 

 (and what little air might be present would doubtless be highly 

 charged with hydrocarbons) would prevent the iron from be- 

 coming more highly oxidized, and cause it to remain still as prot- 

 oxide, as was found to be the case by analysis. 



Pofytelite*, 

 ( Weissgiltigerz, Silberf alder z.) 



Locality. Foxdale silver lead mine, Isle of Manf . 



The occurrence of true silver ores in the United Kingdom is 

 of such rarity as to invest the discovery of the present mineral, 

 especially since it has been found in quantity sufficient to make 

 it an object of commercial consideration, with great interest. 

 Although the cupriferous tetrahedrite (occasionally containing 

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

 localities in both England, Scotland, Ireland, and Wales, there 

 is no analysis of true silver fahlerz or polytelite, or even occur- 

 rence of the mineral itself recorded, as far as the author has been 

 able to ascertain. 



The characters of this mineral are as follows : — massive ; opake; 

 lustre metallic; colour, brown-black; streak black to brown- 



* So many different subspecies are comprised under the denominations 

 tetrahedrite and fahlerz, that the name polytelite (from 7roXireX»)s, valuable), 

 given by Glocker to this mineral, is one which deserves to be retained as 

 being characteristic of the species. 



t The specimen examined was taken from the dressing-floors of this 

 mine, by Mr. Melville Attwood, this summer. Mr. J. L. Thomas, the 

 manager of this mine, has since informed me that this mineral was first 

 observed early this spring, and occurred along with copper pyrites in the 

 workings furthest to the east in the mine, where the lode runs out of the 

 slates into the granite. 



