IRONSTONE FORMATION OF THE FOREST OF DEAN. 



271 



consideration of the petrological conditions under which they are found. 

 By referring back to the arrangement I have sketched at page 225, 

 of the marine carboniferous series (limestone), it will be seen that 

 the ironstone formation caps division No. 3, and is immediately 

 overlaid by the White Head limestone. (See also the section, page 

 226.) The White Head limestone is a regularly stratified rock ; the beds 

 are often highly crystalline, and do not contain, as far as I am aware, 

 any fossils. The limestone rocks and calcareo-argillaceous shales below 

 the ironstone formation comprises the grey and red beds, which are also 

 regularly stratified, but contain, as already described, a moderate quantity 

 of organic remains. I draw attention to this fact of the rocks above and 

 below the ''Mine Measures" being regularly stratified, because in the 

 whole extent of this formation, which has an average vertical thickness 

 of twenty-five yards, not one single instance of lamination or " bedding" 

 has been detected. The entire mass forms a single bed, so to speak, of 

 cavernous limestone, the caverns being filled with the crystallized and 

 powdery haematite-ores which have been described. This limestone is 

 locally called ''crease," and is traversed by innumerable small joints, 

 which exhibit such a total want of symmetry in their arrangement as 

 to render it impossible to attribute their origin to any force acting in 

 any given direction. In one spot, one set of joints will run with sufiicient 

 regularity over a short distance to be easily mistaken for planes of 

 stratification ; at another spot another set will exhibit the same appear- 

 ances, but in a totally different direction ; and both, within a very 

 limited area, will be gradually confused and obscured by a third set 

 cutting them promiscuously. The resemblance in these joints to what 

 may be observed in highly crystalline igneous rocks, such as granite, 

 where they owe their existence probably to an internal shrinking in the 

 consolidation of the mass, is worthy of remark. The worked- out spaces 

 in the " Mine Measures " in which the ore has been removed, and the 

 limestone buttresses, or " crease," alone left standing, forming natural 

 pillars and arches, remind one of the crypt of & cathedral — or, as the 

 caverns occupy somewhat parallel positions in the bed, a series of crypts 

 piled one over the other : indeed, the rude symmetry which in many 

 places is maintained in the position of the buttresses and the regularity 

 of the passages, render it difficult to, at once, disarm the mind of the 

 fact of the non-existence of any artificial means, beyond removing the 

 ore, having been employed in producing the illusion. Connected 



