268 Geological Society. 



rocks, the order of which, commencing with the earliest, is as 

 follows : — 



(a) A felsite with phenocrysts of micropegmatite, and quartz which shows 



good rhombohedral cleavage. 



(b) A series, hereafter called the ' dark igneous rocks.' 



(c) Granitoid veins, subdivided into : 



(1) The Meldon Aplite and its associates ; 



(2) Fine-grained granites of the ordinary Dartmoor type. 



The evidence on which this chronology has been based seems 

 fairly clear. The felsite with micropegmatite occurs as inclusions 

 in the ' dark igneous rocks.' The ' dark igneous rocks ' occur as 

 inclusions in the Meldon Aplite. The Meldon Aplite occurs as veins 

 in the ' dark igneous rocks.' No evidence is available as to the 

 relative age of the Meldon Aplite and the granite-veins. 



A marked feature of the 'dark igneous ' rocks is that they are 

 locally agglomeratic, as such they have been identified as meta- 

 morphosed tuffs. But, on the other hand, every exposure is also 

 ill part homogeneous and compact, with clear flow-structure. The 

 inclusions, where present, are always in part fragments of the 

 contact-rocks of the walls of the sills or dykes. Some of the 

 agglomeratic rocks are certainly d} r kes and not sills, and as such 

 cannot be interbedded tuffs. Ever} r exposure at some place irregu- 

 larly invades the contact-shales. For these and other reasons their 

 identification as tuffs is dismissed, and it is sought to explain the 

 occurrence of the included fragments by successive injections of the 

 same fissures and the break-up of previously-consolidated injected 

 material. 



The geography of the Meldon Aplite is described : it occurs in 

 several dykes, the principal of which extends from east of the 

 western wall of Okehampton Park to the old Ice House on Sourton 

 Tor, a distance of nearly 2 miles. There are other minor dykes 

 north and south of this. 



The texture of the aplite is microgranitic. The principal 

 minerals are albite, orthoclase, microperthite, quartz, lepidolite 

 green tourmaline, and topaz. Blue apatite is almost entitled to 

 be classed with these. Fluorspar, montmorillonite, and axinite are 

 accessories. Although, in conformity with other observers, the 

 author has described this rock as an aplite, he uses the term with 

 reservations. The rock is neither more acid than the normal 

 granite, nor does it approach freedom from mica, and he submits 

 that the true description, even if cumbrous, would be lepidolite-soda- 

 granite. The whole of the mica is apparently lepidolite, and of 

 8'70 per cent., the total of the alkalies, roughly five-eighths are 

 soda and three-eighths potash. 



Some veins of true granite occur, always of fine grain : in these 

 andalusite is locally developed. It is noted that topaz and anda- 

 lusite have never yet been found side by side in any Dartmoor 

 granite or granite-vein, but topaz may occur in granite which is 

 in contact with slate in which andalusite is present. In one and 

 the same rock the minerals appear to be mutually exclusive, or, in 

 other words, when the conditions are such that topaz may form 

 andalusite is not to be expected. 



