IRONSTONE FORMATION OF THE FOREST OF DEAN. 277 



this, much to be traced to that which is mechanical, and still more to that 

 which is purely chemical. The " mine-measures " may have been the 

 result of a rapidly-accumulated calcareous silt, afterwards so altered by 

 contraction and upheaval, as to have had all evidence of lamination 

 and bedding obliterated ; and, from being fissured during elevatory 

 movements, may have become gradually cavernous by the mechanical 

 and solvent powers of water. Into these fissures and hollows water 

 saturated with carbonic acid, and holding proto-carbonate of iron in 

 solution, may have deposited the iron by a gradual escape of part of 

 the carbonic acid, in the same way as in the case of calcareous 

 ^'sinter," when, by the admission of oxygen, it would soon become 

 per-oxidised. Bischoff has shown that oxide of iron may be deposited 

 in cavities, to which air has aocess, in globular and reniform masses 

 having the greatest resemblance to brown haematite, from springs 

 of a temperature of about 91^ Fahr., and in connection with the 

 upheaval of the ''crease "-bed by subterraneous forces, it is more than 

 probable that hot springs may have been produced ; indeed, it is inte- 

 resting to know that such phenomena are not even now wholly absent, 

 and that springs, of a considerably higher temperature than that of 

 water usually met with in not very deep mines, are still encountered 

 in working the " churns." Should we assume the contemporaneity 

 of the ore with the enveloping ''crease," the investigation of the 

 formation becomes of the highest interest in connection with the 

 Paragenesis of minerals."^ But be the origin of these deposits what it 

 may, there can be no doubt but that subsequent chemical agencies 

 causing segregation and re -arrangement of particles, have largely 

 contributed to the condition in which the ore is now found. 



The iron mines are worked by sinking a shaft down through the ore- 

 measures to the " under-stone " in the deepest point of the " award," 

 or, if the nature of the surface admits of such an arrangement, by driv- 

 ing a level in from a valley, so as to intersect the measures as they rise 

 from the centre of the basin. When the churns are reached, " deep- 

 headings " are driven out right and left along the line of " strike " of 

 the bed, and other roads are then driven out at right angles to these main 



The Paragenesis of Minerals (from a Greek verb signifying to be present with 

 TTapayuyvo^ai) or the co-existence and mode of association of mineral 

 bodies, is a matter well-deserving of special study, and destined to throw much 

 light on the history of chemically-formed rocks, and particularly of mineral 

 veins.— J. J.W. W. 



