Physical Structure and older stratified Deposits of Devonshire. 653 



2. — Section from Dartmoor to the Plymouth Limestone, Sfc. 

 (PI. LI., fig. 9.) 



If we cross the country from the south end of Dartmoor to Plymouth, we 

 have a less perfect section than that above described, for the lower calcareous, 

 or Ashburton, bands have almost disappeared. This may be easily accounted 

 for; as even the thickest bands of the limestones, which are interpolated in the 

 slate series, continually thin off; and in this case some of them may have 

 been cut away by the encroachment of the granite. 



Commencing with the boundary of the granite, we first meet with highly altered slates pene- 

 trated by granite veins, and tilted at a great angle. These, through various gradations, pass into 

 the hard slates of Ivy Bridge, which, in the ascending order, finally pass into the ordinary dark 

 lead-coloured slates, exactly like the courses overlying the Ashburton limestone. 



In most parts of South Devon, slates are obtained from planes parallel to the laminae of de- 

 posit ; and, generally speaking, the transverse planes of slaty cleavage are incomparably less 

 developed than among the schistose rocks of North Devon. But in a quarry about two miles 

 south-west of Ugborough, we found very instructive examples of transverse cleavage, the planes 

 of which had the exact strike of the beds, but were inclined at a much greater angle. Other, 

 more inclined beds (both in the same and neighbouring quarries), were gradually brought into a 

 position nearly coincident with that of the slaty structure, and then ceased to show any oblique 

 cleavage planes ; a fact, which clearly shows, that the cleavage planes were superinduced on the 

 beds after their dislocation. We remarked, also, that the best slates were obtained, not from the 

 laminae of deposit, but from the oblique planes. 



Near Ridgeway (on the Plymouth road) are several courses of trap, which range very nearly 

 with the beds ; and a little to the north are quarries of slate, associated with trap. One of them 

 gives some obscure traces of an oblique cleavage, and exposes purple and dark leaden-coloured 

 slates, which are much indurated, and spotted exactly like the altered chiastolite slates of Cum- 

 berland, On the south side of the quarry, close to the tiap, is a chloritic slate, with a few 

 crystals of felspar ; yet this chloritic slate seems, by most regular gradations, to pass into the or- 

 dinary dark-coloured slate. We need not dwell on details familiar to all who have examined the 

 great slaty regions of England, but state at once, that, by a regular dip to the south, the whole 

 slate series is carried under the Plymouth limestone. 



In other respects this and the preceding section agree in their general details. 



The Plymouth limestone is composed of a great succession of thick, well-defined beds, dipping 

 south, at an angle averaging at least 30° or 35°; and as its superficial breadth amounts in some 

 places nearly to a mile, and it is not repeated by any great contortions, its thickness must be very 

 considerable. Notwithstanding this, if we follow it to the east, along the line of strike, it comes to 

 an edge at the distance of about four miles ; and if we follow it in the opposite direction to the 

 Cornish side of the Sound, it thins oflf still more rapidly ; so that before we reach White Sand 

 Bay, it has dwindled down to a few inconsiderable red beds of calcareous sandstone, containing 

 Encrinites and other fossils ; and further along the Cornish coast we lose all traces of it. A few 

 masses of limestone, comparatively of inconsiderable extent and thickness, break out here and 

 there, nearly on the line of strike between Plymouth and Berry Head, and seem to represent, in 



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