480 Mr. Austen on the 



ceous sandstones, earthy hornblendic beds are rendered highly crystalhne, as near 

 Ilsington, and the associated strata abound with garnets ; the development also of 

 mica in beds which previously did not contain any (none occurring at a short di- 

 stance) is very common. Some of the projected granite veins, which are usually 

 composed of a homogeneous rock, hardly crystalline, terminate towards their 

 points in a curious assemblage of plates of mica. 



The old slates have been much altered along the line of the trap dyke near 

 Bickington, N.E. of Ashburton, as may be seen where it crosses the summit of the 

 high hill between that place and Ingsdon ; the slate is also much changed, often 

 passing into jasper. Large blocks of a similar altered rock are to be seen between 

 Bickington and the bridge along the old road ; also about Higher Wear on the 

 Teign. These blocks, derived from beds of old slate affected by heat, are in one 

 instance collected in great numbers, and of large size, along the summit of a ridge 

 of carbonaceous sandstone and conglomerate, particularly at Hestow Farm, near 

 Lindridge. 



Cleavage and Jointed Structure. 



Incidental mention has been made of those great divisional joints which traverse 

 whole formations in given directions, and which are more particularly observable 

 in the slate, limestone, and new red sandstone strata ; besides these, the whole of 

 the older series of South Devon rocks present what have been termed lines of 

 cleavage. 



Our knowledge of this branch of geological inquiry is exceedingly imperfect ; it 

 seems however to be generally supposed that the phsenomena are due to the agency 

 of heat, and an instance has been mentioned (p. 479) in which an intruded mass of 

 heated trap may, on a small scale, have superinduced such a structure ; and in 

 further confirmation of this view, it will be found, that wherever the general ap- 

 pearance of the rocks in any particular spot indicates a greater degree of alteration 

 than elsewhere, that there the subordinate lines of roofing-slate show an increased 

 number of cleavage planes ; and when we note the very great variation which the 

 rocks of the same age may present in this respect, from one place to another at no 

 great distance, we see enough to feel satisfied that such a structure can be no cri- 

 terion of relative geological age. 



The superinduced cleavage structure of South Devon is as follows. In the south-east portions of the 

 county the lines are in the same direction with the planes of deposition, and occasionally coincident, 

 as at the Ingsdon Down and other quarries (PI. XLI.) ; but very often, when at first sight such appears 

 to be the case, a more careful examination shows that one set of lines crosses the other at small 

 angles ; and it is this structure which so frequently renders it difficult to procure perfect specimens of 

 the fossils contained in those slates: thus the cleavage and bedding of the fossiliferous slates of 

 Ogwell would be supposed to conform ; but the cleavage planes are constant, whilst the bedding is waved, 



