472 J. PrestwicHj Esq., on the 



deposition of the lower series in the north of England ; still, from the littoral 

 and fluviatile character of the deposit, it is possible, that its interpolations of 

 freshwater and land detritus with that of the oceanic currents, may have been 

 prolonged during the whole remaining carboniferous era, and even have 

 continued to that of the new red sandstone, for it has been shown that in parts 

 of this district, the coal-measures appear to graduate upwards into the lower 

 new red sandstone ; and Mr. Murchison has proved that in most of the adjoin- 

 ing fields, there is a conformable transition between these two formations. 



Upon the second point, which refers to the nature and effects of the disturb- 

 ances, but little need be said. It has been shown in the observations upon the 

 faults, that the whole of the Coalbrook Dale district has been protruded * 

 through the new red sandstone, and that this angular promontory of up- 

 lifted rocks forms merely the extremity of the principal zone of the Silurian 

 system described by Mr. Murchison. This form of the detached area is due 

 apparently to the intersection of two lines of disturbance. The more im- 

 portant one constitutes the axis of the Wrekin, with its parallel longitudinal 

 dislocations and anticlinal axes, ranging nearly S.W. and N.E. ; the other 

 trends rather N. of N.N.E. ; and the intersection of these two converging 

 lines gives rise to numerous diagonal, connecting dislocations. 



We have no direct evidence of the precise period at which the dislocations 

 were effected. It can only be stated, that they took place subsequent to the 

 deposition of the Lower Red Sandstone system. 



From the details in pages 447 and 457, and from the description of adjacent 

 coal-fields by Mr. Murchison f, it is evident, that the coal-measures of Coal- 

 brook Dale are but a part of an extensive deposit from which they have been 

 disjointed and separated by great subterranean movements. Hence, we may 

 infer, that a portion of that deposit still remains intact below the overlying 

 formations of new red sandstone of Shropshire, Cheshire, and Staffordshire ; 

 and it is not improbable that, by other lines of disturbances traversing these 

 counties, the coal-measures, although covered by the new red sandstone, may 



* I use this word to signify merely the change, by subterranean agency, of the relative level 

 of these two sedimentary deposits, as 1 consider that this effect being due to the contraction of the 

 interior nucleus of the earth, the collapse of the strata resulting therefrom would produce a gene- 

 ral and tolerably regular subsidence of large central areas, accompanied by the elevation, fracture, 

 and tilting of narrow longitudinal segments ; whence the strong lateral pressure evinced in the 

 anticlinal axes and along the lines of fault, and the absence in this disrupted district of open 

 fissures, such as would have resulted from the tension attendant upon an independent expansion 

 of a portion of the interior nucleus. 



t See Mr. Murchison's Silurian System, p. 79, et seq. 



J 



