322 Geological Society : — 



variation or relation to the surrounding rocks ; so that evidently 

 they have not taken up any appreciable portion of the material 

 through which they have broken. The dykes of Group (3) being 

 probably Pre-tertiary (the author does not himself find it possible 

 to distinguish igneous rocks by their geologic age) would be termed 

 melaphyres on the continent ; but those of (2) and (4) are nearer 

 to the group of diabases. The Cleveland dyke (Group 1) is almost 

 certainly of Tertiary age, and its structure and composition entitle 

 it to the name of an augite-andesite. 



3. " The Droitwich Brine Springs and Saliferous Marls." By 

 C. Parkinson, Esq., E.G.S. 



The author referred to the effects of the pumping of brine from 

 beneath Droitwich in producing insecurity in the buildings, and 

 proceeded to discuss the possible source of the brine-water system. 

 He referred to the probable existence of extensive beds of rock-salt, 

 lower than the present brine-cavities, towards the north-east of 

 Droitwich — a conclusion which receives support from the deeper 

 borings carried on at the Stoke works. Full details of these and 

 other recent borings were given by the author. 



January 9, 1884.— J. W. Hulke, Esq., E.E.S., President, 

 in the Chair. 



The following communications were read : — 



1. " On the Volcanic Group of St. Davids." By the Rev. Prof. 

 J. P. Blake, M.A., F.G.S. 



The result of the author's examination of the rocks in the district 

 of St. Davids which have been designated Dimetian, Arvonian, and 

 Pebidian, is that they belong to one volcanic series, whose members 

 are those usually recognized in eruptive areas, and whose age is 

 anterior to, and independent of, the true Cambrian epoch. 



The independence of this series and the Cambrian is shown by the 

 nature of the junction at all points of the circuit that have been 

 seen. In a bay west of Nun's Chapel the junction on the beach is 

 actually a faulted one, the conglomerate being cut out, and the 

 amount of ashy rock seen comparatively small. To the west of 

 this all ashes have been cut out, granite and slate being in 

 contact to the east: at Caerbwdy there is a quarter of a mile 

 across the ashes. This shows discordance. In the Solva valley 

 the beds beneath the conglomerate are again different, and up 

 the higher reach the series on the north and south side are quite 

 distinct, showing a fault. At Trehenliw the conglomerate is absent, 

 at Ogof Goldfa there is a forked fault. South of Castell the conglo- 

 merates and slates strike directly at the consolidated ashes, and at 

 Carn-ar-wig the conglomerate is actually seen overlying unconform- 

 ably green ashes and agglomerates, silky schists being in the neigh- 

 bourhood, but nowhere near the visible junction. At Ogof Iiesugn 



