22 H. C. Sargent — The Penmaenmawr Intrusions. 



were filled from below by a magma depleted of a considerable portion 

 of its more basic elements. In the case of the segregation-patches, 

 we have apparently a local residuum of the more acid constituents 

 separated in the course of the consolidation of the enveloping rock. 

 On the other hand, Rosenbusch suggests that Schaub's analyses of 

 these veins indicate that they are the effect of decomposition rather 

 than differentiation-products. 1 Four analyses are quoted below. 



On Fig. 2, an enlarged sketch-map of Penmaenmawr, the precise 

 spots at which specimens for thin sections were collected are shown, 

 and the specific gravity of each specimen is given. 



It might reasonably be expected that decreasing density from 

 margin to centre would follow the progressive acidity in the same 

 direction noted above, but, as will be seen from the figures, the 

 reverse is on the whole the case. The rock appears to attain its 

 greatest density in the neighbourhood of the summit and in the 

 western quarries directly below. This is no doubt the result of 

 greater concentration locally of the heavier minerals, especially 

 ilmenite, bronzite, and augite, as observed above. 



Alteration of minerals is of course an important factor as affecting 

 their density, which is generally lowered thereby, through the 

 removal in solution of heavy constituents. The variations in the 

 density of specimens collected within a short distance of each other 

 near the summit and in the Graig Lwyd quarries are probably the 

 result of more or less intensive local alteration, coupled with 

 capricious distribution of some of the heavier minerals. 



The compact marginal rock shows a tolerably uniform density 

 ranging from 2732 to 2771. Here too the extensive alteration 

 noted above has probably had a lowering effect, and, as already noted, 

 iron-ores and ferro-magnesian minerals are less abundant than in 

 other parts of the mass. Not always, however, is alteration attended 

 by diminution of density. In the case of the green veins where the 

 felspars are chloritized, the specific gravity generally exceeds 2 90. 

 Seeing that the rock before alteration consisted mainly of quartz and 

 felspar, it is evident that there has here been an increase of density, 

 resulting perhaps from derived iron and magnesia, which it is 

 suggested have been liberated from the bronzite of the enclosing 

 rock on its alteration into bastite. 



Phillips, in his paper referred to above, gives analyses (of which 

 three are quoted below) of four specimens from different parts of the 

 mass, as evidence of progressive metamorphism by surface agencies. 

 The present writer finds it difficult to accept his conclusions in their 

 entirety. Phillips' "practically unaltered rock", which gave the 

 results shown in his No. i analysis, is described as fine-grained and 

 distinctly green in colour. It was evidently very near to the 

 marginal rock with andesitic structure, since it was collected from 



1 "Die von Schaub- mitgeteilten Analysen dieser Schlieren lassen sich 

 kaum als Differentiationsprodukte verstehen, sondern deuten eher auf Zerset- 

 zungserscheinungen " : Mikr. Phys., 4th ed., vol. ii, p. 1258, 1908. [The 

 analyses of the veins, communicated by Schaub, can scarcely be considered 

 differentiation products, but rather suggest that they are the effects of 

 decomposition. — Ed.] 



