Physical Structure and older stratified Deposits of Devonshire. 647 



great angle to the planes of the strata , and hence, where the strata dip to the south, these joints 



o-enerally underhe to the north. This double system of joints divides many parts of the beds into a 



great series of rhombohedral solids. The cleavage-planes, in no instance which we observed, were 



parallel to these joints. Hence, also, it follows, that the jointed structure and fissile structure are 



not to be confounded. Almost all solid rocks have more or less of a jointed structure; but a 



small number of them have a transverse fissile structure. The two structures may, and do often, 



coexist, but, in all descriptions of old rocks, ought to be carefully distinguished from one another. 



Such are the phenomena presented by this group in the only sections in which the successive 



parts are well exhibited. In its range towards the east, it is for many miles very ill exposed ; 



being much covered by the vegetation and alluvial matter near the flanks of the higher ridges 



formed of the harder beds of the second group. Between Timberscombe and Dulverton it crosses 



the road, as above stated, at the water-shed, and contains, at least, three distinct calcareous courses. 



Further east it again declines in elevation ; and the calcareous portion skirting the south flank of 



Croydon Hill is principally represented by one very widely expanded course of limestone, which is 



extensively worked. To the east and north-east of Croydon Hill it is again subdivided into several 



distinct courses of calcareous slate and limestone. These are precisely the phenomena we might 



have expected in such a formation. The structure seems to be most complicated towards its western 



extremity. Assuming the breadth of the formation at a mile and a half, and the average dip at 35°, 



we shall (after deducting for a few local contortions) have a very great thickness for this group. 



Stems of Encrinites and portions of corals are among the abundant fossils of this group. Some 



of the calcareous slates of Ilfracombe are almost covered with the Favosites polymorpha, which is 



so abundant in the Torbay limestone. There are in this group also a Producta, several species 



of Spirifers, and other fossils, to which we shall refer in the sequel. 



4. We have little to add to what we have already said, respecting the structure of the next su- 

 perior group. The lower portions, which abound in soft chloritic slates, exhibit very fine examples 

 of slaty cleavage, as distinguished both from joints and planes of stratification. For example, in 

 the cuttings for the new road in the hills south of Ilfracombe, the cleavage has a nearly uniform 

 dip at a great angle to the S.S.W. or S. by W. ; while the strata, as may be made out from the 

 stripe, are here and there violently contorted. The same facts are still more clearly illustrated 

 on the coast near Morte Point, and in descending into Morte Bay from the north. This part of 

 the series abounds in large, white quartz veins ; several of which aflfect a kind of rude parallelism 

 to the strike of the beds, but others are in undefined oblique directions. They are all to be re- 

 garded as veins of segregation, by some means or other superinduced on the rock, during the 

 passage into its present solid state. 



The upper and more arenaceous portion of this group, occupying the middle and southern end 

 of Morte Bay, is ill exposed on the coast ; but it may be considered to terminate in some thick 

 and very hard beds of gray, greenish gray, and purple-coloured sandstones, alternating with thin 

 beds of chloritic and purple glossy slate. These beds may be traced along their strike through 

 North Buckland, and thence, skirting the south end of the hills containing the old manganese 

 works, towards Muddiford. 



Notwithstanding the many contortions of the chloritic slates, and some undulations (especially 

 as shown in the sections on the road from Dulverton to Minehead) of the upper and harder rocks, 

 the prevailing dip is towards the south ; and the whole thickness of this group is very great, per- 

 haps greater than that of any of the other divisions into which we have separated the deposits, we 

 are describing. In no part of it did we observe any organic remains. 



5. There is no definite line of separation between this and the preceding group : for beds of 



