Reports & Proceedings — Geological Society of London. 41 



Of these, (1) consists of impure grits, which, being well within 

 the aureole surrounding the granite, have developed secondary- 

 mica, a little tourmaline, and small well-formed rutiles. In some 

 places, at contacts with granitoid veins, andalusite is also found. 

 (2) Consists mainly of porcellanites with beds of black chert-like 

 rock. The characteristic mineral of the porcellanites is wollastonite, 

 but at contacts with the Meldon Aplite garnet, idocrase, scapolite, 

 axinite, and lepidolite are also developed. (3) Shows little sign of 

 metamorpliic action. (4) Are cherts of the character already well 

 known as occurring in the Lower Culm-measures, and described by 

 the late Dr. G. J. Hinde & Mr. Howard Fox. (5) Is a dark-grey 

 rock, almost black, the characteristic mineral of which is chiastolite. 

 All these rocks succeed each other conformably, and there is no 

 evidence of folding or repetition. 



In the sedimentary series planes of weakness have developed, the 

 surface traces of which are broadly coincident with the strike, but 

 which frequently lie counter to the dip. These planes have been 

 more or less successfully invaded by at least three series of igneous 

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

 follows : — 



1. A felsite with phenocrysts of micropegmatite, and quartz which shows 



good rhombohedral cleavage. 



2. A series, hereafter called the " dark igneous rocks ". 



3. Granitoid veins, subdivided into — 



(1) The Meldon Aplite and its associates ; 



(2) Fine-grained granite 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 

 in 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 dykes and not sills, and as such 

 cannot be interbedded tuffs. Every exposure at some place 

 irregularly 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. 



