Physical Structure and older stratified Deposits of Devonshire. 655 



stone, and the calcareous slates, partake of all the complicated flexures of the 

 slate series. The contorted beds of Purbeck limestone, associated with tabular 

 masses of Portland rock, are examples of similar phenomena among secondary 

 strata. 



4. In some of the larger masses of limestone, the crystalline action, during- 

 the process of solidification, has almost obliterated every trace of deposit. All 

 of them are more or less traversed by open joints ; and some of the Plymouth 

 beds are intersected by vertical north and south dip-joints of great regularity. 

 The limestones are also occasionally divided into laminse by oblique lines of 

 cleavao-e, like those before noticed near Ilfracombe. Fine examples of this 

 structure occur in several places north of Totness, and in the hills on the east 

 side of Brixham ; and the phenomena are the more remarkable, because an 

 oblique slaty cleavage is very rarely seen among the neighbouring slate rocks. 

 A certain quantity of calcareous matter seems in some instances to favour the 

 development of this structure. For example ; along the whole course of the 

 Dart below Totness, the beds are generally without the least trace of an 

 oblique slaty cleavage; but at East Corn worth Creek, a band of limestone is 

 dislocated, dolomitized, and tilted at a great angle to the north, and is over- 

 laid by some calcareous slates, which, near the beach, dip north, at a small 

 angle, and have a distinct cleavage, inclining south at 45°. An exactly similar 

 phenomenon may be seen in a small mass of calcareous slate imbedded in the 

 great contorted red sandstone south of Plymouth ; and there, also, the neigh- 

 bouring strata are entirely without any transverse cleavage. 



5, Before we quit the subject, we may notice one or two peculiarities of 

 structure which are seen, though rarely, in South Devon, but very abundantly 

 in the calcareous slates of the Cornish coast, west of Padstow. A slight change 

 of colour generally marks the passage from one bed of slate to another; and 

 hence, when several beds are cut through by an oblique cleavage plane, the 

 surface of the slate so obtained is marked by a set of parallel stripes indicating 

 the course of the beds. It is also clear, that slates obtained from laminae par- 

 allel to the bedding can never show a striped surface of the same nature. 

 But some slaty rocks (e. g. on the coast of South Devon, near Avon Mouth, 

 and on the north coast of Cornwall) have what may be called ^ false stripe, 

 which is at first sight very deceptive. They are derived from beds, the fine 

 laminaB of which, in passing into a solid state, have been forced, by crystalline 

 or mechanical forces, into wavy lines or flutings, sometimes nearly constant in 

 their prevailing direction. Slates cleft off from such beds show stripes of co- 

 lour parallel to the flutings, and are to be distinguished from the true stripe 

 only by being worse defined and less uniform in their direction. That these 



