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



tive hardness and ready fusibility before the blowpipe, I am 

 disposed to refer it) it is possibly among the substances hitherto 

 undescribed.* Associated with this series of rocks, we found a 

 variety of compact felspar so penetrated partly by hornblende, 

 and partly by the carbonaceous matter which tinges the lime- 

 stone (C) as to assume a deep-black colour.-)- This is distin- 

 guished from the compact slate A by its more conchoidal 

 fracture, its greater brittleness and fusibility before the blowpipe. 



The limestone is succeeded by dark-grey and black argilla- 

 ceous slate, occasionally passing into a hornblende slate, and 

 containing many subordinate beds, or masses of small grained 

 porphyritic greenstone. In the latter, the imbedded substance 

 is invariably felspar ; the same mineral seems predominant also 

 in the base. J Some specimens procured in Mr. Saville's copse, 

 nearly opposite to Pen Clee Flats, are by no means destitute of 

 beauty. At this spot also my companion § discovered a portion 

 of the black slate containing minute crystals of chiastolite, a 

 mineral which had not before, as far as I am aware, been noticed 

 in the west of England. I have since found it in a similar rock 

 occupying nearly the same relative situation to the granite near 

 Ivy Bridge, on the south of Dartmoor. Passing on towards the 

 new road, cut by Mr. Saville, on the left bank of the river, we 

 found the slate losing its intense black colour, and assuming 

 all the characters of transition clay slate. At the section 

 afforded by the abovementioned road, it is found of various 

 shades of grey and fawn colour, occasionally containing spangles 

 of mica. Here it alternates with beds of compact greywacke,|| 

 and exhibits the curved and undulating stratification so strikingly 

 characteristic of this series throughout its whole extent in the 

 west. 



To this imperfect sketch, I have only to add, that the rock 

 which I have described as greenstone exhibits no traces of 

 hornblende either as an imbedded substance, or in the veins or 

 rifts. As insulated specimens, many or most of its varieties 

 might, with more accuracy, be considered as compact or slaty 

 felspar. Considered, however, in its geological relations with 



* I have found traces of the same mineral and also of axinite in a slaty compact fel- 

 -spar, more or less tinged with hornblende (?) near Ivy Bridge. The specimens of compact 

 felspar from this spot (I. B.) are among the best characterized and handsomest which I 

 observed in the west of England. 



T A rock apparently identical with this occurs on the southern boundary of Dartmoor 

 between the greenstone of Bovey Tracy and the granite. 



% The rock may in general be referred to granite, Div. I. D. c. of Dr. Macculloch's 

 Classif. In some blocks we observed this greenstone apparently traversed by veins of a 

 yet darker and more compact variety much resembling common basalt. 



§ The Rev. P. Serle. 



|| I have used the term greywacke, because, from the general aspect of this portion of 

 rock, most geologists would at once, I apprehend, consider it as belonging to that for- 

 mation. Both the slaty and compact varieties are, however, fusible, though, with diffi- 

 culty, by the blowpipe. The latter is, perhaps, an intimate mixture of granular felspar 

 and quartz. 



