250 
THE GEOLOGIST. 
lines of which the amount of weathering seems to have been greatest, 
and the dolomitic character most brought out. 
The vein of haematite, it will be observed, reposes immediately on 
the uppermost bed of limestone, and is overlaid directly by the con- 
glomerate ; indeed, the one seems to form a bed separating the two 
descriptions of rock, and it might doubtless be assumed to be an 
intei-stratified deposit but for the great irregularity of the bounding 
walls. Moreover, interspersed in the mass, sometimes in contact 
with, but more often perfectly isolated from, the parent rock, large 
fragments of limestone lie bedded in the ore; while large cracks and 
fissures in the conglomerate, partly filled with ironstone, evidently 
prove that the two formations were first torn asunder, and the ore 
subsequently introduced. The ore differs mineralogically from the 
" mine" raised from the carboniferous limestone in other localities 
chiefly by a superabundance of silica, the associated minerals — vein- 
stones — being chiefly quartz, large crystals of which may be found 
lining the angles and cavities in the vein. The magnitude of the 
deposit generally has already been referred to, but not its average 
thickness, which, from wall to wall, may be fairly calculated as not 
less than fifteen feet, and is probably much more as it lies deeper 
from the crop : the engine-shaft (see PI. X.) passes through a mass 
of ore upwards of fourteen yards solid ; but this, although taken perpen- 
dicularly, is of course an oblique measurement as regards the proper 
section of the vein. The siliceous character of the matrix has probably 
influenced the crystallization of the haematite, and the highest per- 
centages are obtained from that description of ore known in the Forest 
of Dean as "flint-brush :" this ore presents a smooth conchoidal fracture, 
and much resembles in colour and surface the appearance presented 
by a freshly broken piece of cast iron. Another kind has a more 
granular fractui'e, possesses a colour between iron-grey and brownish 
red, has a semi-metallic lustre, and is of an exceedingly compact 
texture. A third variety might probably be called Black Hajmatite, 
and, from its affording a violet-coloured glass with borax before the 
blow-pipe, proves itself to be a mixture of iron-ore and manganese : — 
its colour is bluish black, and its lustre imperfectly metallic. All 
kinds are mostly impregnated and interlaminated with quartz, which 
frequently confers a fibrous or semi-ligneous character to the large 
