106 A. B. Hunt — Age of the Dartmoor Granites. 



However, in the case of Dartmoor, there is no necessity to limit 

 the tempei'ature to that of the isogeotherm of the granite itself in 

 post-Carboniferous times ; as, on our hypothesis of the rock being 

 permeable by liquids, the heated gases and liquids may have been 

 derived from much lower levels. 



But to return to the evidence of the associated saline and fresh- 

 water inclusions : these seem to indicate a consolidated granite, 

 subsequently heated, cracked, and permeated by liquids. The 

 question at once arises whether this granite, the original granite 

 of Dartmoor, first consolidated before or after the deposition of the 

 adjacent Devonian and Carboniferous rocks ? If after their de- 

 position, it is inexplicable how a mass of heated, deep-seated granite, 

 supposed by many to be even volcanic in character, twenty miles in 

 diameter, and which as such must have taken centuries to cool, 

 could have had so little effect on the adjacent sedimentaries as to 

 fail in every known instance to obliterate the exact point of contact. 

 If before their deposition, then the granite is pre-Devonian, which 

 is the point of my argument. 



It seems likely enough that a partial aqueo-igneous solution of 

 the ancient Dartmoor granite, accompanied by earth movements in 

 post-Carboniferous times, would have resulted in the injection of the 

 dissolved material into fissures of all kinds, and especially into 

 the main lines of weakness between the granite and the adjacent 

 sedimentaries. Thus all the local contact-alterations, so insufficient 

 if attributed to the action of the main mass of the granite on its 

 pi'imary consolidation, would find an adequate cause in the contact 

 of the newer intrusive or re-constituted granites, which, so far as 

 I am aware, never occur in any considerable mass relatively to the 

 whole crystalline area. 



It would greatly facilitate further research in the Dartmoor area, 

 if the following three points could be definitely decided : — 



(1) Whether the volcanic hypothesis is tenable? 



(2) Whether the chlorides in the quartzes are of marine or plutonic 

 derivation ? 



(3) An explanation of the immediate juxtaposition of brine and 

 fresh-water inclusions. 



It is to be regretted that the problems connected with fluid in- 

 clusions excite at the present time so little interest, even when not 

 treated with absolute contempt. Some of them are perplexing 

 enough. Take for example Trowlesworthite.^ In my slide of this 

 rock a certain crystal of Apatite (identified by Mr. Harker) contains 

 liquid inclusions with small and active bubbles and with no indication 

 of chlorides ; whereas the quartz contains some inclusions with salts 

 and others apparently without. This rock thus presents even greater 

 complications than the granite mentioned by Dr. Sorby in which the 

 felspars contained fresh water and the quartzes brine.' 



At the time that Dr. Sorby delivered his Presidential Address to 

 Section C. at Swansea, it was generally supposed that while super- 



^ See Worth, Trans. Dev. Assoc, vol. xix, p. 494. 

 2 Rep. Brit. Assoc. 1880, p. 670. 



