288 F. RTJTLEY ON THE SCHISTOSE 



schistose rocks of the volcanic series in the vicinity of Brent Tor are 

 merely repetitions of those about Tavistock, ought they not all to be 

 regarded as of late Devonian ago ? and may they not be correlated 

 with those in the Saltash district, which are unquestionably Devonian? 

 That this southern Devonian series of rocks has been elevated by 

 the underlying granitic masses, such as that at Kingston Down, 

 seems to be clearly indicated by the well-marked east-and-west trend 

 of the long elvan dykes which cross the country ; and if this be the 

 case, the discrepancy in the dip of the rocks about Brent Tor and 

 those about Saltash is sufficiently accounted for, while at the same 

 time we can readily realize the denudation of the once intervening 

 and more elevated portions of these schistose rocks over a tolerably 

 wide area. Mr. J. A. Phillips adduces good reason for thinking that 

 some of these lavas and tuifs were restricted to small dimensions ; 

 and he is doubtless right to a certain extent. I cannot, however, 

 help thinking that in other cases they covered a very considerable 

 area ; and I believe that some of the Saltash and Tavistock schistose 

 lavas may possibly have represented a large and once continous 

 flow. I do not, however, wish to urge this opinion very strongly, 

 as I have not visited the Saltash district, but have merely examined 

 one or two specimens collected by Mr. Bauerman. I would also 

 disclaim any desire to remove my neighbours' landmarks or tamper 

 with existing geological boundary lines, when ignorant of all the 

 arguments which may be brought to bear upon such a question, 

 since this paper has been written merely to record a little additional 

 information concerning the nature of the schistose volcanic beds of 

 the Brent-Tor district and to direct the attention of geologists to a 

 very small area in which they may possibly rectify a " scientific 

 frontier " by carrying the Devonian rocks a mile or two further north. 



Before any more really careful work can be done to elucidate the 

 structure of this district and the stratigraphical relations of the 

 eruptive rocks which occur so plentifully in it, it will be necessary 

 to survey it upon a map of larger scale, a proceeding which would 

 prove beneficial to miners as well as to geologists. 



The rocks treated of in this paper are, for the most part, situated, 

 as already mentioned, in the immediate vicinity of Brent Tor ; and 

 it seems more than probable that they emanated from it, as suggested 

 by Sir Henry De la Beche. Having now discussed the characters 

 of these schistose beds of lavas, tuffs, and tufaceous sediments, it 

 may not be out of place to attempt, by a series of sections, based in 

 part upon the Survey Map, to restore the old volcano itself. 



Treating the subject broadly, we may say that Brent Tor stands on 

 an irregular oval basin (fig. 1) occupied by its lavas, tuffs, and ashes, 

 which "are interstratified with sediments hitherto considered to be of 

 Carboniferous age, but which, as I have suggested, may possibly turn 

 out to be Devonian. The basin is cracked across in a nearly north- 

 west and south-east direction by a fault, the downthrow being to the 

 west. Brent Tor is situated on the downcast side of the fault ; and 

 it is to this fact that its preservation is due. The basin-like area is 

 mainly represented by Heathfield. Eurther to the west the beds 



