358 A. Heard— 



to be very slight. In the case of the reaction of potassium iodide 

 and lead nitrate in gelatine, the medium appears to have some 

 distinctly disturbing effect upon the reaction (op. cit., p. 25). The 

 precipitates formed in the experiments cited appeared to be 

 continuous, even to microscopical examination. When the plate 

 on which the reaction had been carried out was placed in a bath of 

 sodium hydroxide solution, a set of bands appeared suddenly in the 

 lead iodide precipitate, and they resembled the vivianite bands under 

 discussion in their apparently irregular spacing and also in the 

 presence of precipitate throughout the spaces between the bands, 

 which also occurs in some cases of the vivianite precipitate in fireclay. 

 It is to be expected that investigation of the effects of the media 

 upon the reactions of solutions diffusing through them will add 

 very considerably to our knowledge of banded precipitates. 



Note on the Llwydmor Bach Granophyre. 



By Albekt Heard, M.Sc, F.G.S., University College of South 

 Wales and Monmouthshire. 



[This paper was submitted to the Editor of the Magazine three 

 weeks before the publication of Mr. N. L. Silvester's paper.^ I 

 commenced work on the area early in the summer of last year, and 

 apparently Mr. Silvester began at about the same time, but as he 

 has secured priority of publication I have decided to postpone 

 further work and publication on the district. — A. H.] 



[Mr, Silvester's paper was in the Editor's hands for some months 

 before publication. It was sent to the printers before the existence 

 of Mr. Heard's MS. was known to the Editor.] 



I. Introduction. 



rPHE rock which is described in this communication, occurs along 

 -*- the western flank of Llwydmor Bach, about 6 miles east- 

 south-east of Bangor, and about 8 miles south-west of Conway. 

 On the 1 in. map of the Geological Survey,^ this rock, together with 

 entirely different types on its eastern margin, are all marked as 

 intrusive felspathic rocks. On the J in. map,^ it is described as 

 granite, and its eastern boundary is mapped. 



The outcrop of the rock is in the form of an elongated lenticular 

 mass, about 3 miles long by | mile wide, with the long axis having 

 a direction nortb-north-east-^south-south-west (approximately). 



In the field the typical rock weathers generally into relatively 

 thin flaggy slabs, which occupy the lower crests of Llwydmor Bach, 

 and form small screes on the slopes. 



Two distinct rock types are present ; the interior of the laccolith 

 consists of a granophyre, whilst near the margin practically all 



1 Geol. Mag., Vol. LTX, 1922, p. 134. 



2 Sheet 78 S.E. Geol. Surv. of England and Wales, Old Series, 1852. 

 ^ Sheets 9 and 10, J in. Geol. Surv. of England and Wales. 



