268 



Professor Sedgwick and Mr. Murchison on the 



dence to rest on, we might infer that the whole coal-field of Liege was inferior to 

 mountain limestone, and the terrain anthraxifere of Dumont ; and this system 

 again inferior to all the slate rocks composing the chain of the Ardennes *. And 

 how are we to rid ourselves of this monstrous conclusion ? 



The answer to this question is given in the details published by Professor Dumont, and to them we 

 must refer f. But we may be permitted briefly to touch upon it, not only as a subject of great import- 

 ance, but as one which admits of very imperfect illustration among the less disturbed sections of En- 

 gland. 



When strata assume a basin-shaped arrangement, we generally find them in this Island dipping towards 

 a centre, which is within the superficial limits of the basin (fig. 10 a.); and a line drawn near the centre of 

 such basin at right angles to the beds composing it would be nearly in a vertical position. Such a line 

 may be called the transverse axis ; and such basins (around which the forces of elevation must have acted 

 with nearly equal intensity) may be called upright basins. But we are under no necessity to limit the 

 forces of elevation (or of subsidence, for the remark applies to both) in this manner ; and we can readily 

 suppose elevatory forces to move the masses on one side through a vertical angle of more than 90° ; 

 while, on the other, the corresponding masses are only moved through a small angle. Still we have a 

 basin-shaped arrangement : the lines of dip converge towards a centre, which is, however, no longer con- 

 fined to the superficial limits of the basin ; and the transverse axis will be more or less inclined to the 

 horizon. Such a one may therefore be called an inclined basin (fig. 10 b.) ; and it is perfectly obvious 

 that vertical sections taken from opposite sides of it will appear directly to contradict one another. 



Fig. 10 a. 



Upright basin. 



Fig. 10 &. 



Inclined basin. 



But, in addition to the vertical sections, every country gives us also, in greater or less perfection, a 

 series of, what we may call, horizontal sections, by help of which the original superposition may often be 

 determined : for it is obvious that, however great and complicated the contortion of any conformable 

 series of strata, they must appear on the horizontal section in the actual juxtaposition they had before 

 their disturbance. Hence they will appear at the actual surface in a kind of symmetrical arrangement, 

 out of which we may determine a true consecutive geological series ; and if we have any means of deter- 

 mining the true relations of the highest or lowest groups, we at once determine the true relations of the 

 whole series. Thus M. Dumont, by the careful examination of natural sections, determined a number 

 of natural mineralogical groups, and following them on their lines of strike, he succeeded in placing them 

 in symmetrical position on the map, or horizontal section. Considered in this way, the coal-field of Liege 

 became only a very complicated case of an oblique basin, or series of basins ; and the same groups of 

 rocks which, on the south of the field, seemed to repose on the coal measures, were seen, on its north 



* See PI. XXIII. fig. 12, where the upper beds are reversed. 



f Memoire sur la Constitution Geologique de la Province de Liege, 4'to, Bruxelles, 1832. 



