572 DISTURBANCES IN SOME TRACTS— QUIET TRANSITION IN OTHERS. 



nating conformably with the strata which were forming, but more often breaking in 

 upon such accumulations subsequent to their consolidation, we must not forget that 

 the younger deposits in this region are comparatively exempted from these signs of 

 igneous outbursts. Within the whole area of the annexed map, there is not a single 

 case of trap in contact with the lias, and only one of a dyke of that matter, cutting 

 through the New Red Sandstone, whilst we have many proofs of volcanic action in 

 the carboniferous strata (affecting also partially the Lower New Red Sandstone), a 

 few evidences only in the Old Red Sandstone, and again, a multitude of eruptions 

 amid the Lower Silurian and Cambrian Systems. It would appear as if, during the 

 ancient periods of submarine accumulation, this region had been periodically much 

 subjected to volcanic action, which, though repeated in subsequent epochs, diminished 

 in the intensity and frequency of its outbursts after the carboniferous sera. This ob- 

 servation, deduced from practical knowledge of the structure of a given tract of country, 

 may indeed be extended to nearly the whole of England, in which there are no traces of 

 trap rock having been intruded into the strata posterior to the period of the oolite, though 

 in Scotland the same action has been most rife subsequent to the formation of the oolites, 

 and in Ireland after the consolidation of the chalk. 



These geological phenomena are strictly in unison with modern observation, which 

 has taught us, that igneous action is often discontinued for long periods in one part of 

 the earth, and called into play in another. We are not, however, to suppose, that 

 when volcanic matter ceased to be protruded, volcanic action ceased to operate, for 

 since then, as we have shown, all the younger secondary and tertiary deposits have 

 been dislocated and upheaved from beneath the sea. The forces which accomplished 

 these great results were, therefore, we suppose the same, as those which evolved the 

 trap rocks at former periods. Intense internal heat acting upon water, and evolving 

 steam and gas, may be imagined to have been a cause adequate even to the elevation of 

 continents ; and it is possible to conceive, how a superincumbent mass of marine sediment, 

 repressing the rise of molten matter to the surface, might, like a high pressure steam ap- 

 paratus, have produced a powerful accumulation of the subterranean gaseous forces 

 which were labouring to expand. But to pass, for the present, from this theoretical 

 point, let us consider a few more facts from which we may legitimately draw our con- 

 clusions. 



In the introduction it was announced, that within this region there were gradual 

 transitions between all the formations which succeed each other. The reader, who has 

 carefully examined the evidences and compared the sections, will find that this assertion 

 has been completely borne out. He may, indeed, think it surprising, that in a country 

 so disturbed by igneous rocks, this unbroken succession of the sedimentary deposits 

 should be visible ; yet he must recollect, on the one hand, that there are wide tracts 

 wholly exempted from disturbance, and, on the other, that without volcanic eruptions, 

 many of the inferior strata would never have been brought to the surface for our ex- 



