1868.] 



MAW — VARIEGATED STBATA. 



393 



Whatever may have been the active agent which brought about these 

 changes of position, the kind of motion which has rearranged the 

 oxide of iron may be suggested by the facts before us. Instead of 

 its direct accumulation into spherical nuclei as in most red beds, 

 the separation seems to have taken place in lines which have ad- 

 vanced in one direction, leaving behind them the bleached sandstone 

 deprived of its iron, and gradually gathering up the iron in its advance. 

 This motion has sometimes taken place in the plane of stratification 

 (Plate Xy. fig. 34), and sometimes centripetally, either towards some 

 mechanical nucleus, which it has ultimately environed and closed over 

 with a ferruginous crust (Plate XY. fig. 33), or in the body of the 

 homogeneous stratum, enclosing, as in fig. 55, p. 390, an isolated 

 portion of the unaltered yellow sandstone. More rarely the motion 

 has taken place centrifugally, the lines expanding from the series of 

 centres (PI. XY.fig. 35, and, supra, fig. 54), ultimately leaving isolated 

 exhausted patches, circumscribed by spherical cakes of oxide of iron, 

 and lying in the midst of the unaltered ferruginous matrix. 



In the Wealden and Keuper sandstone these lines of segregation 

 are often disposed with a very complex arrangement, one ferrugi- 

 nous band abruptly terminating at right angles to a horizontal or 

 concentric series. The lines never cross each other ; and one series 

 appears to have absorbed (as in fig. 36, PL XY.), and rearranged on 

 its own line of disposition, the 



oxide of iron from the lines it Fig. 60. — Yellow-banded Carboni- 

 superseded. ferous Sandstone, Benthall, near 



There are cases, as in fig. Broseley, 

 60, of banded Carboniferous 

 sandstone at Benthall, near 

 Broseley, in which a number 

 of concentric lines of ferru- 

 ginous segregation appear to 

 have simultaneously advanced 

 and closed in towards a centre, 

 producing an alternation of 

 brown bands of accumulation, 

 and bleached bands of deple- 

 tion, the lightest and darkest 

 parts being always in juccta- 

 position. 



Another point to be noticed is the occurrence, within the light 

 areas of depletion, such as are represented in figs. 53, 54, 55, 58, 

 and 61, of isolated patches of the unaltered yellow rock, circum- 

 scribed by the ferruginous line of accumulation. In these cases the 

 ferruginous line bounding the mass of the depleted area is very 

 sinuous, apparently from an irregular rate of advance at different 

 points. The outer yellow ground may thus intersect the depleted 

 area by deep bays (b, fig. 61); and these, by the continued irregular 

 advance outwards of the absorbing line, may become isolated and left 

 behind, like islands (a, fig. 61) in the depleted area. There is also 

 another cause for this phenomenon : the sinuous circumscribing line 



