276 



THE GEOLOGIST. 



run at irregular distances apart, but have a constant bearing of 30'* 

 to 40 » east of north, and in no case do they continue into the over- 

 lying ''crease" of the "mine-measures" proper. The breadth of 

 the joints is often nine and twelve feet, but varying, widening, 

 and then " nipping-in." In connection with the Forest mine- 

 measures, it is interesting to turn to those great deposits of ironstone 

 which are formed in the Carboniferous limestone of the north of England. 

 At Whitehaven the haematite occurs in deposits of irregular dimensions, 

 forming dense masses subdivided by vertical joints, but which do not 

 observe any parallelism. Botryoidal forms of the ore, accompanied by 

 crystallized calcareous minerals, are formed in cavities. Limestone forms 

 the roof, and shales, interbedded with ore, bound the deposit in depth. 

 No satisfactory conclusion as to the origin of these deposits has yet been 

 arrived at ; but the ore most probably occupies cavernous spaces which 

 were produced in the limestone — which differs from the Forest '' crease " 

 in being '' bedded " — by contraction and subsequent water erosion. 

 These haematites yield 60 per cent, of per-oxide of iron, united with from 

 5 to 6 per cent, of silica, and with minute quantities of alumina, lime, 

 magnesia, and occasionally manganese. By referring to the section, 

 page 267, it will be seen that a deposit of ironstone occurs in the lower- 

 most division of the Millstone Grit. This deposit, which is known as 

 the "sandstone-vein," does not "make itself" with the same regu- 

 larity as the "ironstone-mine," nor is it of the same practical 

 importance, since although the ore is, on the average, of a richer 

 quality, the churns are never of the same magnitude. The whole 

 thickness of the bed through which the churns run seldom exceeds five 

 feet ; but, as yet, this vein has been explored only to a very small 

 extent, and it is more than possible that its economic pretensions will 

 rise when it comes to be further worked. In nearly all respects the 

 churns of the " sandstone-vein " agree in their general structure, and 

 in the mode in which they are filled, with the deposits in the limestone ; 

 but it is more easy, in their case, to become convinced of the real 

 contemporaneity of the ore with the containing rock, than in the case 

 of the "limestone-vein." 



The origin and mode of formation of the " iron-measures " in the 

 Forest of Dean is certainly involved in much obscurity, occurring as 

 they do, in the case of the limestone deposits, in an unstratified bed be- 

 tween stratified rocks. There is doubtless, however, in the cause of 



