on the Physical Structure of Devonshire. 3 1 3 



bands. Near the fossiliferous schists the beds become calcareous, 

 and in one place pass into an impure limestone : many of the beds 

 have a slaty cleavage transverse to the stratification, and cutting through 

 the non-calcareous portions and the lines of organic remains. The 

 thickness of this group is great, but its base is not exposed. 



2. The preceding division passes by almost insensiblegradations into 

 the great red arenaceous groups already mentioned. The beds of green- 

 ish slate, shillot, &c, become quite subordinate, and the whole cha- 

 racter of the formation is derived from the coarse arenaceous beds, 

 sometimes passing into a siliceous conglomerate. These coarser 

 beds are commonly red or variegated j among them, however, are grey 

 and greenish grey beds, the colours, as might be expected, being- 

 inconstant. Oxide of iron traverses some portions of these rocks in 

 thin veins, and that mineral abounds so much in some of the beds, 

 that they have been regularly quarried (e. g. near Comb Martin 

 and to the south-east of Porlock), and smelted in the iron foundries. 

 The slaty cleavage transverse to the bedding almost disappears among 

 these rocks, but they are much intersected by joints, some quite ir- 

 regular in bearing; but two sets, one ranging with the beds and the 

 other transverse to these (respectively called strike joints and dip 

 joints), are described to be of common occurrence. The authors found 

 no organic remains in this group, but they have been found, though 

 very rarely, in some of the shillots and finer schistose masses, which 

 are subordinate to the coarser red siliceous sandstones. Its whole 

 thickness is computed (especially from the coast section west of the 

 Valley of Rocks) to be five or six thousand feet. 



3. The next group differs greatly from the former, in having compa- 

 ratively few of the coarse siliceous sandstones, wanting the red 

 colour ; abounding in bands of rotten slate, sometimes like dark in- 

 durated slate, but more frequently greenish and chloritic, and 

 commonly exhibiting a cleavage distinct from the stratification. 

 It also contains many calcareous bands (in some places not less than 

 eight or nine), which occasionally swell out into masses of limestone, 

 and numerous organic remains, not however generally well pre- 

 served. Its thickness is estimated at two or three thousand feet, 

 and notwithstanding some contortions, it dips on the whole to- 

 wards the south : its strike, like that of the beds near the coast, 

 is about east-south-east and west-north-west. . This formation 

 is traced far into the interior, and is identified with the calcareous 

 system flanking the Quantock Hills. 



4. The next group has the same strike as the preceding, and is of 

 enormous thickness, though not so great as might at first sight be 

 imagined from its breadth or the surface of the country and its high 

 inclination, as many parts of it are violently contorted. The authors 

 divide it into two portions, the lower abounding in fine glossy chlo- 

 ritic schist, much contorted, and having a true cleavage transverse 

 to the bed, and generally presenting a succession of parallel fissile 

 planes, dipping at a high angle to the south j the upper beds con- 

 taining similar masses alternating with coarse, thick, arenaceous 

 bands, some of which resemble the rocks of the second group of the 

 section. 



Third Series. Vol. 11. No. 67. Sept. 1837. 2 S 



