ORIGIN OF THE ANCIENT SEDIMENTARY DEPOSITS. 



573 



animation. In those districts, where igneous masses have been suddenly intruded at 

 one point, and have as suddenly ceased to be evolved at others, the strata in the dis- 

 turbed, when compared with those in the undisturbed districts, are necessarily found 

 in unconformable positions. From this cause, the Silurian strata of Shropshire fold 

 round the inclined and disrupted edges of the Cambrian Rocks, showing that these had 

 been set on edge anterior to the accumulation of the younger deposits ; whilst in Caer- 

 marthenshire, and partially at the Berwyn mountains, the links not being broken off, 

 there are actual passages from the one system into the other. In Shropshire, the in- 

 clined position of the oldest deposits terminates abruptly with the disappearance of the 

 intrusive rocks, and the younger strata of the tract, undulating in less disturbed 

 masses, are necessarily unconformable to the older. In Caermarthenshire, all the sedi- 

 mentary masses have been more or less conjointly elevated, thus demonstrating, that 

 whilst in certain districts accumulations have been forming for a length of time, in un- 

 disturbed sequence, in other tracts their continuity has been destroyed by dislocations 

 or volcanic eruptions. 



If, from the facts before us, we attempt to unravel such problems, how much more 

 difficult is the task of retracing in imagination the outline of the surface of the earth 

 during the production of these varied deposits ! Whence, it may be asked, was the 

 detritus derived, which, layer after layer, was piled up in such stupendous masses 

 beneath the then existing seas ? However incomprehensible it may appear to those 

 who have not studied the subject, geologists entertain no doubt that all our present 

 mountains composed of sedimentary matter, were accumulated beneath the sea during 

 countless ages ; and, if so, other continents must have existed to furnish materials, 

 though no traces of such lands now remain. 



Again, how wonderful is the uniformity in mineral character which pervades such 

 masses over large spaces ! The black shale and schist, the red, grey and purple 

 sandstone of the Silurian and Cambrian Systems, were once, as already stated, nothing 

 more than the finely comminuted mud and sand which were spread out successively over 

 the beds of great estuaries ; — but how vast must have been the streams through which 

 they were diffused ! The united powers of transport of the Ganges and the Amazon 

 must, indeed, have been in action during immense periods, to deposit masses of sedi- 

 ment so extensive, as those which we know were formed at that epoch in Europe, 

 Africa, and America. 



If the scale of these operations be surprising, equally so are the changes in the suc- 

 cession of deposits. The sediments poured forth to form the Cambrian and Silurian 

 Systems, indicate by their structure and colour, either that the lands whence they 

 were derived successively changed their outline, some portions being elevated and 

 others depressed, or that the affluent streams of the great transporting channels were 

 from time to time directed into new directions. Ascending from these ancient strata, 

 the whole geological series, consisting of alternations of mud, sand, and lime, attests 



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