270 



THE GEOLOGIST. 



argillaceous character, and contain from 30 to 34 per cent, of iron, giving 

 off water to the amount of from 32 to 35 per cent, of their weight by 

 torrefaction. 



But, by far the most important produce of the Forest are the Brown 

 Hgematite ores, or hydrated per-oxides, which occur in such vast quan- 

 tities, in what are called the " Mine Measures," and which, as already 

 described, occupy the upper beds of the carboniferous limestone and the 

 lower beds of the millstone grit. The diagram, page 226, shows a 

 section of the *'Mine Measures," and the section, page 267, shows the 

 complete sequence of the beds in connection with the overlying Coal 

 Measures. 



The ores are usually classified into 1, Brush ; " 2, " Smith Mine ; ^' 

 and 3, **Clod." — 1. The Brush-ore is a hsematite, containing as much as 80 

 per cent, of iron, when pure; is compact, of a submetallic lustre, botryoi- 

 dal, kidney-shaped, formed of concentric coats, and having a fibrous or 

 radiated structure. More often, however, it is stalactitical, and covered 

 with brilliant black crystals of hydrous oxide of iron, which are often 

 so minute and so thickly aggregated as to confer a velvetty appearance of 

 great beauty. The local term " Brush " is probably derived from this 

 stalactitic form. 



2. The Smith-ore consists of finely powdered per-oxide of iron^ 

 "which occurs so pure as to be raised almost entirely free from 

 extraneous minerals ; it usually contains from 54 to 58 per cent of iron. 



3. The Clod is a marl highly charged with the per-oxide of iron^ 

 and with small fragments of compact hsematite or brush." It is found 

 under particular circumstances, to which I shall presently allude, and 

 is valuable to a certain extent as a ''flux" from the contained alumnia. 



The following are some analyses made by me of (average) samples of 

 Forest of Dean ores 



No. 1. No. 2. No. 3. No. 4. 



Per-oxide of Iron 



62 



70 



80 



86 



Carbonate of Lime 



10 



0 



3 



0 



Silica 



6 



12 



6 



10 



Alumina 



4 



0 



0 



0 



Carbonate of Magnesia 



2 



4 



0 



0 



Manganese 



2 



0 



1 



0 



Water and Loss 



14 



14 



10 



4 





100 



100* 



100 



lOOf 



From this mineralogical description of the ores, let us turn to the 

 * From Sandstone Vein. f Black Brush Ore, 



