Physical Structure and older stratified Deposits of Devonshire. 677 



Upper Culm-Measures. 



This group, essentially composed of sandstone and indurated shale, is ex- 

 panded through all the undescribed portions of the great culm trough, and 

 indeed occupies nine-tenths of its whole surface. It is of very great, but un- 

 ascertained, thickness ; being perpetually interrupted, coiled upon itself, and 

 repeated over again, by violent contortions, forming an incredible number of 

 anticlinal and synclinal lines, ranging nearly east and west with the strike 

 of the beds*. No words can exaggerate the number and violence of these 



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broken at their points of contrary flexure, and exhibiting a succession of 

 cusps, like regular pointed arches — sometimes, though more rarely, thrown 

 into salient and re-entering angles, generally of local extent and only affect- 

 ing particular beds. Indeed, we have some examples of such angles in thick 

 beds of shale, where the massy sandstones, above and below them, are not at 

 all contorted t- 



These contortions become less violent as we approach the lower groups on 

 the southern and northern outskirts of the formation. Thus, on leaving the 

 lower group near Oakhampton and making a traverse towards the north, we 

 cross, for some miles, a great ascending series of the upper culm group, in 

 which the prevailing dip is north, and in which there are comparatively few 

 undulations. And in like manner, in a traverse from the northern boundary 

 (for example along either of the roads from Barnstaple to Bideford), we have 

 a great ascending series of beds, and a prevailing dip to the south. Hence, 

 though it be impossible to ascertain the whole thickness of the group, we are 

 sure that it is very great ; and respecting the general order of superposition 

 we find no difficulty. Whatever may be the age of the upper culm series, it 

 is uncontestably the highest group of our general north and south sections;};. 



We have stated that the essential components of this group are sandstone and shale ; some- 

 times exhibited in numerous thin bands, sometimes in thick well-defined masses. The sandstones 

 are generally close grained, and full of irregular joints; and are coated over with ferruginous stains, 



* Near the eastern boundary of the formation (as has been already stated) these lines are more 

 irregular in their direction ; and, as might be expected, there are many subordinate tracts (gene- 

 rally, however, of small extent) where the beds are in inextricable confusion, and their strike is 

 irreducible to any system. 



t A curious example of this kind (noticed in Mr. Greenough's map,) occurs at Mill Hook, on 

 the coast of Cornwall. 



t See the sections from the Foreland to Dartmoor, and from Marwood to South Petherwin. 

 (Pi. LI., figs. 1 & 2.) 



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