646 Professor Sedgwick and R. I. Murchison, Esq., on the 



careous nor fossiliferous ; but by these words, we mean to state only that we found no fossils in 

 it, and that at least they are very rare. We must remember, however, that it is interpolated in a 

 fossiliferous system ; and that a better search might probably bring to light a few fossils among 

 the finer strata, alternating with the coarser red sandstones. 



3. The next group of calcareous and fossiliferous slate is of very complicated mineral structure. 

 The first beds are coarse and siliceous, and contain fragments of large, heart-shaped, bivalve shells. 

 These are succeeded by clay-slate, more or less arenaceous, all dipping, as before, about S.S.W., 

 and at a considerable angle. Ascending by the valley of Combe Martin to Yellaton (a distance 

 of about a mile and a half), we cross not less than eight or nine courses of highly calcareous slate, 

 in some places passing into great bunches of bluish crystalline limestone, with many contempora- 

 neous veins. The limestones have some traces of organic remains, which occur also in abundance, 

 but generally as casts, in the neighbouring calcareous slate. One of these calcareous courses 

 immediately south of Combe Martin, is not less than 60 feet thick. In general, they are much 

 thinner; and none of them are very continuous in the direction of the beds; but contracting to an 

 edge, and expanding, several times, along the line of strike, they occasionally appear more like 

 huge lenticular masses of semi-crystalline limestone, interpolated irregularly among the cal- 

 careous slates, than true alternating beds. Four or five of these calcareous courses have been 

 traced, by our friend Major Harding, through the headland which separates Combe Martin from 

 Ilfracombe. 



The cliffs on both sides of Ilfracombe Harbour are interesting, from their great complexity of 

 structure. Some of them are made up of alternating strata of hard siliceous sandstone ; and beds 

 of soft, glossy, yellowish green, chloritic slate : others are composed of dark-coloured clay-slate, 

 with quartz veins, alternating with hard quartzose beds. Some parts are devoid of calcareous 

 matter ; others abound in it ; and among the earthy slates, west of the harbour, are thin bands of 

 limestone, almost made up of encrinital stems. Among the calcareous slates, we find quartz veins, 

 sometimes ranging in long stripes, nearly parallel to the beds ; sometimes in irregular zig-zags 

 nearly transverse to them ; Bnd in one or two instances thin veins of arragonite range nearly 

 parallel to the strike. In the cliffs west of the harbour, where the structure is thus complicated, 

 the beds are violently contorted : still further west, they acquire the prevailing dip (S.S.W.), by 

 which the distinct courses of calcareous slate, with great lenticular masses of limestone, are 

 brought down to the coast ; and the whole mass plunges under the next higher group. 



The phenomena of slaty cleavage may be studied among these cliffs to great advantage. The 

 average dip of the beds is about 35° to the S.S.W. ; and among all the more fissile beds, we have 

 a cleavage dipping to the same point, but at a greater angle ; perhaps on the average as much as 

 60° or 70°. These cleavages affect also the calcareous beds, and make it almost impossible to 

 ascertain their true dip in the quarries, where the contiguous strata are not exposed. The cleavage 

 planes are not merely posterior to the beds, but are of a date posterior to those movements which 

 produced the contortions : for there are several places, where these planes pass, without deviation 

 in their strike and inclination, through a succession of violently-contorted beds. We may remark, 

 also, that when any part of these beds is brought into such a position, that the cleavage becomes 

 a tangent plane, the fissile texture will then coincide with the true bedding of the rock, and again 

 separate from it, as the contorted beds deviate from the unvarying cleavage course. 



All the neighbouring coast is intersected by joints, often forming a succession of great parallel 

 open fissures. Of such there are two sets, — one to which we give the name of dip-joints is 

 nearly transverse to the strike, and most frequently in a vertical or very highly-inclined position ; 

 the other set (or strike-joints) is nearly coincident with the strike, and generally inclined at a 



