100 A. R. Hunt — Age of the Dartmoor Granites. 



suggestion, I submitted the question to the British Association at 

 Leeds. The following extract is from the published abstract of my 

 paper : — 



" The theory of the marine origin of the saline inclusions in the 

 Dartmoor rocks seems to harmonize well with the view commonly 

 entertained that the chlorine and chloride of sodium emitted by 

 volcanoes are derived from the sea.' 



" In the case of volcanoes the presence of hydrogen and chlorine 

 may be accounted for by the dissociation of the water and of the 

 chloride of sodium by the intense heat, and the combination of the 

 two gases thus formed would result in the production of hydro- 

 chloric acid. 



" In the case of the cooler granites there is no question of dis- 

 sociation and of gases, but of the entanglement of brine and steam 

 at more moderate temperatures. 



"Thus the access of salt water to highly heated rocks seems to 

 account for some of the more important gases emitted by lavas 

 and of the more characteristic fluid inclusions caught up by granites. 



" An alternative theory, that the crystals of salt in the Dartmoor 

 rocks 'had been formed from hydrochloric acid gas acting on the soda 

 in the rocks,' does not seem to the author to account for the crystals 

 in the quartz-veins of the culm slates, or to explain the complete 

 permeation of the granite by the chloride of sodium. Moreover, 

 the one theory accounts for the presence and origin of the hydro- 

 chloric acid as well as of the soda, whereas the other has to assume 

 the previous existence of soda and the advent of hydrochloric acid 

 from unknown quarters." '^ 



The vei-y title of this paper was omitted from the newspaper 

 reports, it Iseing dismissed as being merely of local interest ! 



Mr. Ussher'e last paper referring to Dartmoor appeared in 1892, 

 and General Mc'Mabon's in 1893. In the discussion which ensued on 

 the reading of the latter. Professor Bonney is reported to have said 

 that " In his opinion Mr. Ussher's theory was quite untenable. If 

 the fusion of a peripheral portion of the Dartmoor mass was due to 

 crushing .... Was there any evidence that a rock could be fused 

 by pressure alone, any more than by a gentle stewing in sea- water 

 vphich also had been suggested ? . . . No good was done for science 

 by proposing hypotheses which, in avoiding one difficulty, raised a 

 number of others far more formidable."^ On the same occasion 

 General Mc'Mahon said that " the word used by Mr. Ussher was 

 ' fusion,' and it was applied to the results of the N. and S. squeeze on 

 the rigid and obstructing pre-Devonian rocks." * 



There seems great misapprehension hei'e. So far from Mr. Ussher 

 attributing fusion of the Dartmoor granite to " pressure" or "crush- 

 ing," or to a " N. and S. squeeze," he seems never even to have used 

 the terms " pressure " or " crushing " in reference to the granite, 

 and his only suggestion of a possible source of heat appears to be 

 " a rise of the isogeotherms or plutonic action " (a quotation from 



1 See Characteristics of Volcanoes, J. D. Dana, p. 8. 

 2 Eeport Brit. Assoc. 1890, p. 815. » Q.J.G.S. vol. xlix. p. 397. * Loc. cit. p. 397. 



