26 H . G. Sargent — The Penmaenmawr Intrusions. 



of volcanic agency, or perhaps actually squeezed out by this crust- 

 pressure itself." x 



If we may assume that the three intrusions discussed in this paper 

 are the plugs of a small group of extinct volcanoes, the observed 

 phenomena would seem capable of ready explanation. 



The lighter and more acid portion of the contents of the inter- 

 crustal reservoir would be first drained off, and it is perhaps 

 represented by the rhyolites of the Dwygyfylchi and T Drosgl series 

 which are distributed round the suggested vents as a centre. 2 



If we assign Penmaenmawr and its neighbouring intrusions as the 

 source of these lavas, the necessity of postulating an earlier volcano 

 in the vicinity of Y Foel Fras, which " has been destroyed by a later 

 and more extensive invasion of the igneous magma", 3 seems to be 

 removed. Later on, the eastern lobe of the Penmaenmawr outcrop, 

 including the Clip yr Orsedd and Graig Lwyd ridge, would be 

 extruded, and this rock would thus constitute a true andesite, from 

 which indeed it is indistinguishable in microscopic structure. It 

 may perhaps be thus paralleled with the andesites of T Foel Fras. 

 Finally, but not necessarily at the same stage in each case, the vents 

 would be choked with the rock forming the higher part of 

 Penmaenmawr and the two smaller intrusions on the south. 



It may be remarked that the boss-like forms of the outcrops, and 

 the behaviour of the surrounding slates, suggest that the three masses 

 rise more or less perpendicularly from below ; and finally, they are 

 aligned with the extinct vents lying to the south-west, Y Foel Fras 

 and Mynydd Mawr, on the strike of the axes of disturbance that 

 produced the folding and the cleavage of the district, and led to the 

 outpouring of the lavas of Bala age. 



PS. — It will be noted that, in the analyses of the rock, soda is 

 always in excess of potash, thus confirming the result of the 

 microscopic examination that orthoclase is subordinate to plagioclase. 

 In the grey veins, which, as stated above, are comparatively fresh 

 and in which plagioclase is subordinate, potash is in excess. In the 

 green veins, where alteration of the felspars has proceeded much 

 further than in the grey veins, the alkalies are strikingly reduced in 

 quantity. 



The high CaO content of the veins, as compared with the rock, is 

 noteworthy. In the former case it appears that the lime has not 

 been removed by alteration-processes (as has probably happened in 

 the case of the rock), but has gone to the formation of prehnite, 

 and its percentage is relatively increased owing to the removal of 

 alkalies, etc. 



It is to be observed that Schaub shows no combined H 2 in his 

 analysis of the green vein (E"o. iv), notwithstanding the presence of 

 hydrated silicates (prehnite and chlorite). Clearly the percentages 

 of this analysis are relatively increased by the omission. 



1 The Bala Volcanic Series of Caernarvonshire, 1889, p. 120. 



2 The writer hopes to describe this series of lavas in detail in a future paper. 



3 Harker, op. cit., p. 124. 



