398 PKOCEEDINGS OF THE GEOLOGICAl SOCIETY. [April 22, 



shire beds are due to segregation, in which carbonate of iron has in 

 like manner been drawn towards centres of aggregation, whilst the 

 hydrous sesquioxide has been dispersed, in the form of receding 

 bands, out of an original matrix containing both the carbonate of 

 protoxide, and sesquioxide of iron in association. The phosphoric 

 acid appears to have been nearly all aggregated with the carbonate 

 of iron, scarcely any occurring in the brown cakes of sesquioxide ; 

 and the carbonic acid has also been almost entirely withdrawn from 

 the brown bands and general mass of stratum to the centres of 

 aggregation of the protoxide of iron. It may here be noticed that 

 concretions of carbonate of iron, wherever occurring, invariably 

 present the uniformly homogeneous structure * observed in those of 

 the Northamptonshire beds, whilst nodules of sesquioxide of iron 

 are characterized by a concentric banding, such as would be the 

 result of successive accumulations of the concentrating lines ob- 

 served in yellow-banded sandstones (fig. 60, p. 393). 



Kernel-roasting. — The artificial process known as " kernel-roast- 

 ing " of copper ores presents some phenomena so closely resembling 

 the mode of aggregation of the oxides of iron in yellow-banded sand- 

 stones, that a brief reference to it may not be inapplicable as illus- 

 trating the kind of motion which the iron appears to have taken. A 

 full description will be found at p. 349 of ^Percy's Metallurgy,' 

 from which the following is abridged. When cupriferous iron py- 

 rites containing, say, from one to two per cent, of copper, in lumps 

 about as large as the fist, is subject to a very gradual roasting at a 

 low heat with access of air, it is found that a large portion of the 

 copper becomes concentrated in the centre of each lump (fig. 63 1). 



Fig. 63. — " Kernel-roasting " of Copper Ores. 

 I 



In the early part of the process, a lump broken across {a and h) 

 consists of a central mass of unchanged ore, enclosed in a shell of a 

 reddish-brown substance like sesquioxide of iron ; and between the 

 two is interposed a thin, more or less continuous layer containing 

 more copper than the original ore. At about the niiddle of the 

 roasting, several such concentric layers may be observed ; and when 

 the process is further advanced, a nucleus of unchanged ore can no 

 longer be seen, the outer brown crust becomes greatly increased, 

 and the concentric stratification of the copper layers is still visible. 



* Merely as regards the absence of banding, as subsequent brecciation has 

 frequently produced complex modifications in the original structure. 



t For the use of this engraving I am indebted to the kindness of Dr. Perev. 



