part 4] sTt'Dr of TitE ST. AtsiEtL GHa^ite. 575 



within the positive mica-areas does not necessarily indicate a causal 

 connexion ; but the concentration of mineralizers has. nevertheless, 

 had its effect upon the type of mica. I have already pointed out 

 that the lithionite of the Hensbarrow area is sometimes so bleached 

 as to be nearly or quite colourless in thin sections ; and such 

 bleached lithionites occur along the areas rich in pneumatolvtic 

 minerals, at Hensbarrow Beacon ; at Trenance ; and near Dubber's 

 Works. Finally, the St. Stephen's area is occupied by an especially 

 rich zone of fluorine-minerals, and here the mica is entirely of the 

 colourless gilbertite type, doubtless closely allied to, and developed 

 from, the lithionite. The effect of the mineralizers is apparently 

 to retain the silicates of iron in solution, and ultimately perhaps to 

 remove them from the granite and deposit them later in mineral 

 veins. 



There remain two other phenomena, especially characteristic of 

 the St. Stephen 1 s area : namely, the late separation of an apatite 

 tilled with inclusions, and the occurrence of nearly pure albite as 

 the plagioclase. It has been shown that all the lime present in 

 bulk analysis is sufficient to account for the phosphoric acid and 

 part of the fluorine only, and it is suggested that the mineralizers 

 have operated on the lime in much the same way as upon the iron. 

 They have retained it in solution so that none was available for 

 the formation of anorthite ; the temperature of separation of 

 apatite was lowered, and the remainder was dejwsited as fluorite 

 in replacement in the very last stages of consolidation, or still later. 

 The general evenness of grain in the St. Stephen's area, and its 

 somewhat greater size, when compared with the lithionite areas, 

 may also be due to the further concentration of mineralizers in the 

 district. 



In short, the history of the intrusion of this granite-mass, so far 

 as the data permit it to be deciphered, begins in the east with the 

 intrusion of a magma partly crystallized, and already containing 

 large perthite - crystals, but still possessing a relatively high 

 temperature and involving a relatively long time-interval before 

 the completion of crystallization. The mica-zoning of the perthites 

 probably indicates the period of injection. The perthite -plates 

 carried up in a central stream were largely suspended at the point 

 where the velocity was checked by deflection. Crystallization again 

 slowly proceeded, both by addition to old crystals and by the 

 initiation of new centres, and a coarse-grained igneous rock resulted. 

 Towards the later stages the country on the east was gradually and 

 progressively invaded, and the few occurrences of biotite-granite may 

 ha vu formed the commencement of the'; invasion. The new intrusive 

 material had advanced farther along the course of crystallization, 

 and to such an extent that some of the quartz had already begun 

 to separate. At contacts with relatively cold country-rock rapid 

 crystallization of the already cooled residual liquor took place, 

 embedding the earlier crystals in an aplitic ground-mass ; while, in 

 the more central parts, the further crystallization proceeded rather 



