1823.] Geology of Devon and Cornwall. 189 



near St. Austle, and of another tor (the name of which I have 

 lost), overtopping the upper road, which leads to Buckland on 

 the Moor. If my memory does not fail me, 1 observed it on a 

 scale equally large in more than one other spot on the Cornish 

 Downs, and on Dartmoor ; but I either neglected to mark at the 

 time, or have since mislaid the exact localities. I would venture 

 to recommend to the examination of future tourists two points 

 connected with this rock. 



1. Whether some other of the tors on the confines of the gra- 

 nitic tracts (besides those specified) do not consist of shorlrock. 

 From its external configuration and neighbourhood to the 

 oranite, it may be easily confounded with that rock until exa- 

 mined more closely. 



2. Is it possible from the careful examination of the country 

 surrounding Roche Hocks, to ascertain whether that remarkable 

 eminence has been produced by the disintegration and washing 

 away of some less durable beds which once enveloped it? or 

 whether it may be regarded as an original inequality of surface?* 



Most of the varieties of external appearance produced in this 

 rock by the different modes of aggregation, have been enume- 

 rated by Mr. Sedgwick (p. 18). To his list, I am enabled to 

 add, as Nos. 5 and 6, 



5. Alternate layers generally of great tenuity, of very minutely 

 granular quartz and shod, having all the aspect of a stratified 

 mass. 



6. Real or pseudo breccia consisting of small patches of 

 compact shorl imbedded in quartz, or vice versa of quartz imbed- 

 ded in shorl. 



These varieties occur in the neighbourhood of the crystalline 

 and more predominant form. No. 5 appears to offer another 

 exemplication of a law, which I believe to obtain pretty gene- 

 rally, " that crystalline rocks when they occur in large masses 

 are most usually accompanied by schistose rocks composed of 

 the same mineral ingredients in a state of greater alternation." 

 To the verification of this law, I would venture to solicit the 

 attention of those who join a knowledge of mineralogy and 

 chemistry to that of geology. The establishment of its probabi- 

 lity might lead to some important theoretical results. Nor 

 should I omit to mention, that the varieties of shorl rock above 

 noticed are peculiarly interesting from the examples which they 

 afford, even in hand specimens, of the various phenomena of 

 configuration incident to rocks of the schistose character. The 

 marked distinction of colour and aspect existing between the 

 two constituents (tourmaline and quartz) render these very strik- 

 ing and intelligible. Even the very small collection which I 

 possess myself, offers within the scale of a few inches highly 

 instructive examples of contortion, dislocation, crossing, and 



* I would suggest a like examination of two singularly insulated masses of rock 

 occupying opposite sides of a ravine near Catnelford, known by the name of the Devil's 

 Leap. 



