298 PROCEEDINGS OF THE AMERICAN ACADEMY. 



departs very widely from this li^aire, wliereas it is 'the latter that 

 should correspond most nearly to the entectic composition on the 

 basis of the theory. It is of course by no means necessary, in a 

 magma representing, like the present one, a very complicated chemical 

 system in which there are certainly a considerable number of as yet 

 imperfectly understood solid-solution relations existing between the 

 various components, that the end product of differentiation should 

 be a eutectic mixture. In any case the figures above given serve to 

 illustrate the present uncertainties of the eutectic theory as applied 

 to such systems. 



Fluorine is present usually in small amount: locally near quartz 

 veins, in the pegmatitic pipes, and in certain contact facies of the 

 porphyry cover it is quite abundant in the form of fluorite, which is its 

 usual mode of combination. Small amounts are present in the rie- 

 beckitic hornV)lende, and in the pegmatite pipes it is also present in 

 the mineral parasite. Zirconia is generally present in small amount as 

 zircon. Titanium is present in ilmenite, aenigmatite, astrophyllite, 

 titanite (in part or wholly secondary) and probably to a small extent 

 in the hornblende. The cerium earths make their appearance in the 

 parasite of the Pegmatites and are doubtless present elsewhere in the 

 granites, etc. Though very small in amount their presence is inter- 

 esting and is probably, like zirconia, characteristic of this type of 

 rock. Traces of molybdenivmi (as molybdenite), lead (galena), zinc 

 (sphalerite) are present in the magma since they are found in the 

 quartz veins and pegmatitic phases. 



Although it is the writer's opinion that the greater part of the 

 original mineralizers were retained within the granite by the quick 

 chilling of the upper zones, these acting as a protecting cover, it is 

 possible, if not indeed probable, that the more volatile contents of the 

 extreme upper portions of the invading magma were in great part lost 

 b}' rapid and easy diffusion through the relatively thin cover of 

 sediments. The occasional strong development of fluorite in certain 

 localities where the indications are, that the rock as now exposed was 

 originally not far removed from an original contact, supports this 

 view. The retention of mineralizers in the magma appears to be a 

 characteristic feature of other occurrences of this type of rock, partic- 

 ularly that of Dobrogea on the Danube which seems to resemble 

 the present occurrence in many respects. ^^ 



The riebeckite of the fine-granite appears to correspond closely to 



siMurgoci, op. cit., pp. 142-144. 



