394 PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [April 22, 



of two neighbouring areas of depletion, in their centrifugal advance, 

 will ultimately coalesce (as near ^. ^^ -r^, . . -r . ^ „ 

 the bottom of fig. 37, Plate XY.) ^'S- Ql.—Directionof Line of Fer- 

 at the first point of contact, and ruginousAocumulationmlanded 

 thus isolate portions of the yel- 2/^^?^^ Sandstones. 

 low ferruginous ground, as in 

 fig. 61, c, c. 



The further encroachment of 

 the line of accumulation over the 

 detached patch will then be con- 

 vergent, though it is simply an 

 uninterrupted continuation of 

 the divergent course of the main 

 line. This may perhaps help to 

 explain the apparently opposite 

 courses that the absorbing lines have taken under similar conditions ; 

 the kind of motion is the same in both cases, the uniform tendency 

 being for the line of accumulation to advance from the already ex- 

 hausted area towards the unexhausted part, whether it he a centrifugal 

 or centripetal motion. 



This yellow banding, represented in fig. 36, Plate XY., is so evi- 

 dently connected with the contiguity of mineral veius, that it may 

 be important to notice the precise manner of its occurrence. The 

 Lower Keuper of Shropshire consists, for the most part, of light-buff 

 sandstone, brown-mottled sandstone (fig. 40, Plate XY.), before re- 

 ferred to (p. 376), red sandstone, scarcely distinguishable from the 

 underlying Bunter, and the cuiiously banded yellow sandstone, 

 fig. 36 (PI. XY.). These do not occupy distinct stratigraphical 

 horizons. The red and buff series of rocks is vertically intersected 

 by copper-lodes* of a bright sea-green colour, for the most part 

 devoid of iron. The sandstone, which at a distance from the lode is 

 red and buff, is, where it forms its boundary, charged with hydrous 

 sesquioxide of iron, arranged in fine bands. The ferruginous lines 

 end irregularly and abruptly against the lode-like mass of sandstone 

 charged with copper, the boundary being generally defined by a thin 

 brown line of hydrous sesquioxide of iron, into which the other ferru- 

 ginous courses coalesce. The presence of this " corduroy rock " is 

 looked upon by the miners as a sure indication of proximity to the 

 copper ; and as the lode is horizontally receded from, the sandstone 

 assumes its ordinary red-and-cream colour. 



Looking, then, at the fact that the light beds of the Keuper are both 

 underlain and succeeded by great masses of red beds, and that they 

 include isolated patches of red rock vertically disposed through their 

 mass, and much iron iu a variety of conditions and modes of ar- 

 rangement on every horizon, their uniformly primordial red colour 

 seems probable ; and the evident connexion between the variegation 



* These are not true lodes with a distinct filling to the matrir, but consist of 

 the ordinary sandstone charged with from 1 to 5 per cent, of carbonate of copper, 

 and containing also cobalt, manganese, baryta, lead, &c., the whole having a lode- 

 like disposition vertically intersecting the adjacent rock. 



