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 access, 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 witli 
Trapaytyvo/xai) or tlie 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. 
