201 



ment of the Iron Carbonate by the Peroxide. This was, almost 

 certainly, effected by means of the infiltration of the same ferriferous 

 solutions that stained the associated sandstones their present deep 

 red tint. The Froghall Ironstone of Staffordshire affords another 

 example of the same nature. In this a band of clay carbonate of 

 iron has been partially, and, in some parts, entirely, altered into 

 haematite. The alteration in this latter case is to be attributed to 

 the same causes that have developed the haematitic character in 

 the Argill Coal Field, namely, the percolation of water charged 

 with iron in solution, derived from the New Red Rocks before 

 they were denuded from that part. 



Professor Warington Smyth is of opinion that the formation of 

 Siderite, at least in the .\lston district, is going on at the present 

 day, as Mr. Attwood found a rod of malleable-iron partly coated 

 with this mineral, which must therefore be, in this case, of later- 

 formation than the date of introduction of the iron rod into the 

 mine (see Q. J. Science, V. p. 36). Professor Smyth adds the 

 remark that the Siderite " invests, as a crystalline encrustation, the 

 previously-fornj^d crystals of Fluor spar and of Galena ; and the 

 striking manner in which it is found to coat only those surfaces 

 which face in a particular direction is well worthy of attention in 

 the study of these obscure phenomena." 



Alstonite, or Bromlite, the Double Carbonate of Lime and 

 Barytes, is one of the minerals that are, so far as our present 

 knowledge extends, confined to one locality. Alstonite is found 

 at Fallowfield, as well as at Bromley Hill, and some of the other 

 mines near to Alston. It has not yet been found in any other 

 part of the world. In appearance some specimens of this mineral 

 bear a certain superficial resernblance to Aragonite, as it crystallizes 

 in forms belonging to the same system, and has much the same 

 lustre and colour. More commonly, however, it is found in the 

 form of groups of small, snow-white, acute pyramids, generally 

 about a quarter of an inch from base to apex. Frequently these 

 pyramids are doubled, base to base. These occur lining cavities 

 in the rider, generally in the parts of the vein that traverse beds of 



