H. C. Sargent — The Penmaenmawr Intrusions. 23 



the most westerly quarry and showed under the microscope a ground- 

 mass of felspar microlites with very little free quartz. The distinctly 

 green colour is perhaps enough in itself to make this specimen 

 suspect, and it has been shown above that alteration has made 

 considerable progress in the marginal rock, especially in regard to 

 the felspar and bronzite phenocrysts. Further, the high specific 

 gravity (2 - 94) of this specimen suggests tbe possibility of alteration 

 of the sort referred to in the case of the green veins. 



The progressive increase of Si 2 in Phillips' analyses appears to be 

 attributed by him, in some degree at any rate, to its liberation from 

 felspar as a result of alteration. In addition to his analyses, two 

 others are given below which Mr. Eric Sinkinson, of the Imperial 

 College of Science and Technology, has kindly prepared for this 

 paper. Of these, one is of the compact rock on the margin of the 

 intrusion, and the other is of the medium-grained rock from the 

 centre. Alteration of some, at any rate, of the constituent minerals 

 has without doubt affected the result of all these analyses, and 

 probably none of them can be taken to represent the original 

 unaltered rock. 



The five analyses are arranged in progressive order to show the 

 composition from margin to centre, and Phillips' analyses are also 

 given in his order of progressive alteration. The writer submits 

 that, looking at the analyses as a whole, they serve to confirm the 

 microscopic examination, and that, instead of progressive alteration, 

 they show that consolidation has to some extent followed the normal 

 course of increasing acidity from the margin inwards. The proportion 

 of secondary quartz seen in thin sections, which may have been 

 liberated from the felspars, etc., is so small as not to interfere with 

 this view. 



The varying amounts of lime and magnesia may probably, to 

 a considerable extent, be attributed to a capricious 'distribution of 

 plagioclase and pyroxene, which is very noticeable in thin sections. 

 This suggestion is perhaps confirmed by the low specific gravity of 

 the specimens which the analyses show to be poorest in these 

 constituents. On the other hand, it is evident that the large 

 percentage of combined water in Phillips' No. iv analysis points to 

 considerable alteration and the formation of hydrated silicates. 



It is, however, clear from what has been said above that, in spite 

 of the progressive acidity referred to, the course of ' fractional 

 crystallization' has not proceeded normally. Bronzite, always the 

 earliest formed and the most basic of the phenocrysts, is more 

 abundant at the close of consolidation than at its commencement. 

 The same observation holds good of both augite and apatite. It is 

 perhaps safest not to include the iron-ores in this consideration, 

 as they probably separate out at any stage of the process, but the 

 rock is essentially more basic in the interior than at the margin. 



Leaving, however, the consideration of this problem for the 

 moment, it now remains to deal with the two small southern 

 intrusions forming the mountains of Dinas and Carregfawr. _ No 

 quarrying operations have been undertaken on Dinas, and it is 

 therefore only the surface rock that can be examined and described. 



