104 A. R. Runt — Age of the Dartmoor Grcuiites. 



It will be observed that the same type of liquid inclusions, in 

 similar variety, can be traced from the simple quartz vein, through 

 various injected or infiltrated compound veins, to the ordinary 

 granite of Dartmoor. 



Description of Fig. IV. : — 



(1) Liquid inclusions in a quartz-vein in Culm slate in the valley 

 below Houndtor Wood, south of the Becka brook. 



(2) liiquid inclusions in the quartz of Trowlesworthite. 



(3) Inclusions with one and two liquids in a granite (No. 20) 

 trawled in English Channel (for comparison). 



(4) Liquid inclusions from normal porphyritic Dartmoor granite, 

 Heytor Quarries. 



(5) Liquid inclusions in a crystal of Apatite enclosed in the 

 quartz of Trowlesworthite (No. 2). The Apatite contains plain 

 fluid ; the quartz, water and cubic crystals. 



All magnified according to linear scale of —dVo i'lch as given. 



The following description of a contact slide between coai'se 

 gi'anite and a micro-granite vein in Lustleigh Cleave applies with 

 equal accuracy to the Ditsworthy vein figured above : — 



(1) Irregular inclusion with cube; hexagon with cube; rhomb 

 with cube. 



(2) Irregular inclusion with cube and bubble ; hexagon with 

 cube and bubble. 



(3) Iri'egular inclusion with bubble; hexagon with bubble; gas 

 inclusion. 



The chief absent variety here is one present in the Ditsworthy 

 rock, viz., the simple fluid inclusion without cube or bubble; it may 

 possibly have been present and been overlooked. 



These extreme variations are all of them intelligible in the case 

 of the veinstones, as we may regard the fissures, in which the latter 

 were crystallized, as tubes capable of resisting any pressure. Then 

 with volcanic heats below and cooler regions above, and with the 

 fissures charged with saline and acidulated waters, we seem to have 

 all the apparatus ready prepared for a laboratory experiment on a 

 grand scale, calculated to produce the phenomena we now see in the 

 resultant veinstones. But how are we to account for these identical 

 phenomena when occasionally reproduced in the main mass of the 

 granite ? Is it possible to apply the explanation that meets the case 

 of the veins, or is there any alternative explanation forthcoming ? 



On the assumption that Dartmoor as a whole was, in Carboni- 

 ferous times, a liquid or viscid magma, how are we to explain the 

 differences of pressure and of solubility of chlorides, indicated 

 throughout almost every cubic inch of its mass? No explanation 

 has ever been offered, geologists having found it far more easy to 

 minimise the importance of the facts than to account for them. 



It is usualljf admitted that the tourmaline of Dartmoor is commonly 

 a secondary mineral ; or, as its pi-esence implies the introduction of 

 two new elements into the gi-anite (fluorine and boron), it is perhaps 

 more accurately described as an imported mineral. Assuming the 



