162 Rev. Mr. Conybeare on the Geology of the [Sept. 



whole Dartmoor range. It is white, and coarse-grained. It 

 contains occasionally veins of felspar, quartz, and tourmalin, and 

 exhibits, though sparingly, the remarkable contemporaneous 

 patches of a finer grain, which have been noticed in other places, 

 and might at first sight be mistaken for imbedded masses. In 

 one of these, I observed an intermixture of extremely minute 

 portions of tourmalin, and a tendency to orbicular aggregation, 

 which, had the rock been less friable, and its formation more 

 perfect, might have afforded specimens for the lapidary. In 

 another block, I noticed a remarkable disposition of quartz and 

 felspar in alternate layers, of about one-fourth of an inch broad, 

 each retaining their usual semicrystalline aspect. This variety 

 evidently passed into the common form of granite. The lami- 

 nated portions (if I may so call them) contained little or no mica. 

 The immediate junction of the granite with the superincumbent 

 rock is concealed on the banks of the river by vegetation, and in 

 its bed by the accumulation of bowlders. Among these, how- 

 ever, are found portions of the same obscure gneiss, formed 

 apparently by the union of a dark violet coloured slaty felspar, 

 with a small portion of mica (or chlorit?) and quartz, which 

 occurs in many parts of the Ocrynian groupe, immediately repos- 

 ing on granite.* It is probable, therefore, that this is the case 

 hi the course either of the Okement, or of one of its tributary 

 streams. The first rock, however, which can be traced in situ 

 (A) is a hard compact black slate, not very readily fissile, yet 

 exhibiting in parts a decidedly laminated structure. Its fracture 

 in the more massive varieties is imperfectly conchoidal, and the 

 fragments thick, under the hammer it is rather tough than 

 brittle, and, before the blowpipe, fuses reluctantly into a nearly 

 opaque glass, of a muddy-white tinged with green. A high 

 magnifying power shows minute brilliant specks, apparently of 

 mica, disseminated through its mass. It is rather crushed than 

 abraded by a common file, and acts as a good touchstone for copper 

 and silver. I have been thus particular in describing this rock, 

 because I am somewhat uncertain whether to ascribe it to the 

 greenstone series which follows, or to consider it (which I am 

 rather disposed to do) as an indurated and massive variety of the 

 clay slate, or killas, which usually reposes on the granite of the 

 west.f This rock is traversed in every direction by granitic and 

 felspathic veins, varying in breadth from some feet to a mere 



* See Prof. Sedgwick's Memoir on Cornwall (Cambridge Phil. Trans, p. 1 12, 113). 

 At the southern extremity of Dartmoor, I have noticed this rock near Ivy Bridge, at 

 fiuckland in the Moor, and especially in the channel of a small stream which flows into 

 Holme Chase a little above the road to Spitchwick. It is found, traversed by small 

 veins of felspar and tourmalin, and forming from its colour and perfection, handsome 

 and well characterized specimens, at a mine sunk upon the point of junction at Kith 

 Hill, near Callington. I have ventured to use the term Ocrynian on the authority of 

 Messrs. Greenough and W. D. Conybeare. 



•f I should refer it to Division I, B. 6, Argil. Schist of Dr. Macculloch's Clas- 

 sification, p. 355 



