PENGELLT — 07* THE AGE OF THE DAllTMOOlt GRANITES. 15 



thing granite which occurs in the true Dartmoor country. Sir Henry 

 De la Beche says, " The granite of Dartmoor is, as a whole, a coarse- 

 grained mixture of quartz, felspar, and mica, the latter sometimes 

 white, at others black, the two micas occasionally occurring in the 

 same mass. It is very frequently porphyritic, from the presence of 

 large crystals of felspar, and here and there schorlaceous ; but the 

 latter character is chiefly confined to the outskirts, w T here the Dart- 

 moor granite adjoins the slates. The schorl not unfrequently occurs 

 in radiating nests of variable size and abundance. A complete pas- 

 sage may generally be traced between the compound of schorl and 

 quartz, usually termed schorl rock, and the ordinary granite. The 

 mica usually disappears as the schorl begins to be abundant, and 

 sometimes, though not very commonly beyond limited areas, the 

 granite is a mixture of mica, schorl, felspar, and quartz, in nearly 

 equal proportions. After the absence of mica the next mineral 

 which commonly disappears is the felspar, leaving the compound 

 a mixture of schorl and quartz, the former sometimes occurring m 

 radiating nests in the latter ; but more commonly the two minerals 

 form an aggregate in nearly equal proportions." * 



This, though a comprehensive, is by no means an exhaustive de- 

 scription ; considerable dissimilarity exists in the size of the aggre- 

 gated crystals in different specimens ; nodules, apparently segregative, 

 sometimes occurring in the substance of the ordinary granite, might, 

 from the fineness of their grain, be almost mistaken for sandstone ; 

 indeed, I not long since heard them appealed to as proofs of the 

 metamorphic origin of granite. " Here," said the appellant, " are un- 

 altered remnants of the old sandstone rocks, which, with these 

 exceptions, metamorphism has converted into granite." I do not 

 quote this for the purpose of endorsing it, but simply to show the 

 general dissimilarity of the nodules to granite proper. Excepting 

 their darker colour, they reminded me much of the granite veins 

 which pass through the older granite of Goatfell, in the Isle of 

 Arran ; nevertheless they are not veins but nodules, and capable of 

 being extracted, as such, from the granitic mass containing them. 

 Good examples of such nodules may be seen, amongst other places, 

 at Shaptor. about two miles from Bovey Tracey, where I succeeded 

 in extracting two good specimens. They consist of very fine grains 

 of quartz and schorl, in about equal quantities, or with the latter 

 somewhat preponderating. 



The observer who enters a Dartmoor quarry soon discovers that 

 granite is by no means weatherproof; the effect of the weather is 

 very discernible, fully a foot or more within the exposed surfaces ; a 

 more or less dark or ferruginous-looking band, of about the width 

 just mentioned, graduates into the unchanged rock, and suggests 

 that small fragments might, through long exposure and rough usage, 

 undergo a very considerable change of aspect. 



The boulders which occur so abundantly in the beds of the Dart- 

 moor rivers and rivulets are found to be more or less changed in 



* " Report," p. 157. 



