THE 



GEOLOGICAL MAGAZINE 



NEW SERIES. DECADE IV. VOL. I. 



No. III.— MARCH, 1894. 



OI^xc3-I3:^^JLIJ .A.I^TICLES. 



L — Four Theories op the Age and Origin of the Dartmoor 



Granites. 

 By A. R. Hunt, M.A. 



FOUR Theories of the Age and Origin of the Dartmoor Granites 

 have been lately current, viz. : — 



(1) The ordinary Plutonic of Sir H. de la Beche. 



(2) The Laccolitic- Platonic of Mr. W. A. E. Ussher.^ 



(3) The Volcanic of Mr. R. N. Worth.^ 



The above all assume the exclusively post-Carboniferous age of 

 the granite, owing to its intrusion into the adjacent Carboniferous 

 rocks ; one of the best attested facts in geology. 



(4) The combined pre-Devonian non-intrusive and post-Carboni- 

 ferous intrusive (both plutonic), advanced by myself in 1889.^ 



Mr. Ussher has recently withdrawn his provisional* laccolitic 

 hypothesis, on stratigraphical grounds, in favour of one which is 

 practically equivalent to No. 4.* 



As it is impossible to compress the work of some four years into 

 a magazine article, the present paper must be confined to a sketch of 

 my main argument. 



My friend Mr. Worth has, with characteristic generosity, lent me 

 the diagrammatic woodcut illustrating his ideal volcano ; accompanied 

 by the following remark : " Please do not limit your views of me ; 

 you owe it to yourself to make your case as strong as possible, and 

 we shall not differ outside our theories." A too generous opponent 

 indeed, and one hard to oppose. 



The following woodcut was published to illustrate Mr. Worth's 

 paper, "The Dartmoor Volcano," Trans. Plymouth Inst. 1888-89. 



Mr. Worth brought his theory in the plainest language before the 

 Geological Society in 1889, when it was discussed by our leading 

 petrologists, apparently accepted by Prof. Bonney and Mr. Hudleston, 

 and not objected to on principle by anyone.'' 



Prima facie, however, there seems a serious objection to Mr. 

 Worth's volcanic hypothesis, as it involves the "Elevation-crater 

 theory " pure and simple.' 



1 Trans. Devon. Assoc, vol. xx. p. 154. 



* " The Dartmoor Volcano," Trans. Plymouth Institution, etc., 1888-89. 



•' Trans. Devon. Assoc, vol. xxi. p. 238. * Trans. Devon. Assoc, vol. xx. p. 156. 



^ Proc. Som. ArchEeological See. vol. xxxviii. p. 204. 



6 Q. J.G.S. vol. xlvi. pp. 80-82. 



' See Geikie, Text Book of Geology, pp. 241, 242. 



[The publication of this article has been unavoidably delayed. — Edit. Geol. Mag.] 



decade IV. — VOL. I.— NO. III. 7 



