Mountains and Tablelands. 323 



of the Silurian rocks in North Wales are of a slaty 

 character, interbedded with masses of hard igneous 

 rocks, which attain in some instances a thickness of 

 thousands of feet. It is, therefore, easy to understand 

 how it happens that with disturbed and contorted beds 

 of such various kinds, those great denudations, which 

 commenced as early as the close of the Lower Silurian 

 period, and have been continued intermittently ever 

 since, through periods of time so immense that the 

 mind refuses to grapple with them it is, I repeat, 

 easily seen how the outlines of the country have assumed 

 such varied and rugged outlines, as those which North 

 Wales, and in a less degree parts of North Pembroke- 

 shire, Devon, and Cornwall, now present. 



I have said that the Secondary and Lower Tertiary 

 strata have not been anywhere disturbed nearly to the 

 same extent as the Palaeozoic formations in England. 

 Though occasionally traversed by faults, yet with rare 

 exceptions most of the strata have been elevated above 

 the water without much bending or contortion on a 

 large scale. What chiefly took place was a slight up- 

 tilting of the strata to the west, which, therefore, all 

 through the centre of England, dip as a whole slightly 

 but steadily to the east and south-east. This is evident 

 from the circumstance that on the Cotswold Hills the 

 lowest Oolitic formation (Inferior Oolite, No. 9, fig. 57) 

 forms the western edge of the tableland, while, in spite 

 of a few minor escarpments that rise on the surface 

 of the upper plain, the uppermost Oolitic beds that dip 

 below the Cretaceous strata, are sometimes at a lower 

 level than the Inferior Oolite at the edge of the plateau. 



The great result, then, of the disturbance and denu- 

 dation of the Palaeozoic strata, and of the lesser dis- 

 turbance and denudation of the Secondary rocks, is, 



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