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



wavy lines were produced by tension_, seems probable when we follow their 

 course, and see it marked by minute joints, filled generally with quartz. We 

 never saw an example of this false stripe in any roofing-slate of Wales or 

 Cumberland ; and the two kinds of stripe very seldom occur together. There 

 are slates also from South Devon and the north coast of Cornwall, which give 

 clear indications of a second cleavage plane affecting the elementary structure 

 of the rock. We speak of a structure never, as far as we know, seen in the old 

 Cambrian slates, and entirely distinct from the small double joints which so 

 often separate individual beds into rhombohedral solids. This second cleavage 

 is indicated by innumerable strias (looking as if made by a sharp instrument 

 like a very fine comb) running through the chloritic flakes that separate the 

 laminations. With care, a cleavage (generally with a rough surface) may be 

 obtained along these striae, which occur indifferently both with the false and 

 true stripe above described. The crystalline forces producing these striae 

 have also produced mechanical tension, as the striae may be sometimes fol- 

 lowed into minute partings, generally filled with quartz. On the river above 

 Padstow are some beautiful examples of these complicated structures. A low 

 cliff shows a set of contorted beds of calcareous slate traced by lines of organic 

 remains : the beds are cut by oblique cleavage planes, producing striped 

 slates. There is a second cleavage indicated by the parallel strije, and the 

 whole mass is divided into rhombohedral solids by a double set of open 

 joints, which do not coincide either with the striep or the stripes. 



6. Lastly, before we proceed to the other sections, we may briefly notice 

 the organic remains of the South Devon limestone. The corals in the Tor 

 Bay, Newton Bushel, and Plymouth limestones, are very abundant ; some of 

 the species, more especially the beautiful feather-like coral {Favosites poly- 

 morpha), abounding at all the places above mentioned. Several species of 

 Spirifers, TerehratulcE, and other shells, bivalve and univalve, are, we believe, 

 common to them all, as might be expected among beds nearly of the same 

 age; but we purpose to return to this subject, when we have better specific 

 evidence. Some of the red coraline, calcareous slates of South Devon cannot 

 be distinguished, either by structure or fossils, from the calcareous slates near 

 Ilfracombe. 



3. — Coast Section from the Plymouth Limestone to BoltHead*. (PI. LI. fig. 5.) 



1. We commence with this section, as offering the clearest evidence of superposition, and the 

 best exposure of the successive beds; and after describing it, we shall briefly notice the corre- 



* The section is supposed to be made on a straight line ; to which the phenomena on the actual 



