H. C. Sargent — The Penmaenmawr Intrusions. 15 



A closely corresponding bed has been identified at Scratcbells Bay, 

 Isle of Wight, in spite of the very small area and unfavourable 

 condition of the section (see the September number of this Magazine, 

 p. 408) ; and the existence of a corresponding bed in Hants is fairly 

 certain from the presence of one of the peculiar plates, with 

 a brachial, at the base of my pit No. 885, * and from various instances 

 of brachials unaccompanied by plates of Marsupites at points obviously 

 on the border-line between the zones of Marsupites and Offaster pilula 

 (notably my pit 2 No. 857). It is therefore likely that such a bed 

 occurs uniformly at this horizon in South England, though the 

 opportunities for putting this to a conclusive test away from cliff 

 sections may be very few. There is hardly a single pit, e.g., in all 

 Hants where the upper beds of the chalk with Marsupites can be 

 positively identified and the succeeding beds are well exposed. 



For a long time this Bed 4 was the lowest point to which I had 

 traced any instance of the free-growing forms of Retispinopora 

 (R. arbusculum, Bryd., and R. patula, Bryd.), or of Lophidiaster 

 pygmceus, Spencer, and I had an uncomfortable feeling that this 

 coincidence was a possible argument against including the bed in the 

 zone of Marsupites ; but this doubt has recently been dispersed by 

 a specimen of R. patula found at the base of the zone of Marsupites 

 ( = Marsupites-band of Bowe) in Kent. 



Beds 6, 7, and 8 I continue to assign to the zone of 0. pilula and 

 subzone of E. scutatus, var. depressus. 



The zone of Marsupites here is almost as prone as the subzone 

 above it to develop flint tabulars, mostly interstratified but never 

 really persistent. 



IV. — The Penmaenmawr Intrusions. 

 By H. C. Sargent, F.G.S. 



THE subject of this paper is a group of three intrusive masses of 

 igneous rock, possibly laccolitic in their origin, whose outcrops 

 are situated within a radius of a mile from the village of Llanfair- 

 fechan, on the north coast of Carnarvonshire. 



The most northerly and the largest of these intrusions is the well- 

 known Penmaenmawr Mountain (1,550 feet above O.D.), and the 

 two others lying to the south are Dinas (1,000 feet) and Carregfawr 

 (1,167 feet). 



The petrology of Penmaenmawr has already been dealt with by 

 several writers of eminence. The rock has been described by Phillips 

 as a quartziferous diorite ; 3 by Teall as an enstatite-diorite ; 4 by 

 Harker as in part a bronzite-bearing quartz-dolerite and in part 

 a quartz-andesite ; 5 by Hatch as markfieldite ; 6 by Bosenbusch as 



1 Brydone, The Stratigraphy of the Chalk of Hants, p. 88 ; London, 

 Dulau & Co., Ltd., 1912. 



2 Op. cit. , p. 86 and (under erroneous number 859) p. 92. 



3 " On the Chemical and Mineralogical Changes which have taken place in 

 certain Eruptive Kocks of North Wales " : Q.J.G.S., vol. xxxiii, pp. 423-9, 1877. 



4 British Petrography, 1888, p. 273. 



5 The Bala Volcanic Series of Caernarvonshire, 1889, p. 64. 



6 The Petrology of the Igneous Bocks, 7th ed., 1914, p. 369. 



