Physical Structure and older stratified Deposits of Devonshire. 669 



The general absence of transverse slaty cleavage, in the parts of the coun- 

 ties we are comparing; the occasional appearance of that structure, especially 

 among the calcareous portions; the occasional appearance of a second oblique 

 cleavage plane ; systems of dip-joints and strike-joints changing with the 

 change of strike ; and many other common accidents of structure (not to 

 mention mineral veins, elvans, contemporaneous trappean rocks, &c.&c.), may 

 also be considered collectively to bear out our previous conclusion. We 

 have thus cleared the way for a description of the great overlying culm-mea- 

 sures of the central region of Devon. 



Chap. IV. — Culmiferous Series. Its relation to the other Formations, Struc- 

 ture, Fossils, ^c. (Plate LI. fig. 1, 2, 3, & 4.) 



We have before stated that this series occupies a great trough, the axis of 

 which ranges nearly east and west; and we have also indicated its north and 

 south boundary in general terms. A formation may, however, be deposited 

 in a trough, and yet the beds on its opposite sides may not exactly correspond. 

 But in this instance there is no such ambiguity ; for the bottom beds of the 

 series have a peculiar mineral structure and are characterized by great beds 

 of black calcareous shale and black limestone, with peculiar fossils, and these 

 appear on both sides of the great trough. 



That these lower beds rest on the highest group (No. 5.) of our northern 

 section we have already given abundant proof They are overlaid by an 

 enormous thickness of contorted beds of shale and sandstone ; and, by a re- 

 versal of the dip, are finally brought out again to the surface and seen resting 

 on the slate rocks of Cornwall and Devon, and on the northern flank of Dart- 

 moor. The trough is therefore symmetrical, and the true bottom beds of the 

 whole series may be studied in detail either on its northern or southern boun- 

 dary. We have, therefore, a most unequivocal proof that the culm-measures 

 are newer than any of the stratified groups associated with them. 



It may be contended, that there is a passage from the base of the culm- 

 measures into the old slate rocks of Cornwall and Devon ; and there may well 

 be a raineralogical passage where both formations alternate with trap, or ap- 

 proach the granite, and so become metamorphic : but passages of structure 

 proving a real continuity of deposit must always be difficult to establish among 

 old slaty rocks, where similar beds recur at such enormous intervals. We 

 must, however, allow that dark slaty beds and indurated shales occur in seve- 

 ral places (for example, near South Petherwin ; in a mining work on a moor 



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