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 

 interstratified 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 PL 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 hsematite, 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 fracture, 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 Haematite, 

 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 



