(5) The local passage of diabase into a chloritic schist in consequence of 



a deformation of the rock mass. 



(6) The existence of numerous more or less vertical dykes some of 



which must be referred to a later period than the intrusive sheets. 

 With regard to the age of the sheets, it seems probable that they be- 

 long on the whole to the same period as the associated lavas of more acid 

 composition. If this be so, the sheets of Merionethshire would belong to 

 the Arenig, and those of the Snowdonian area to the Middle Bala period. 



Mid- Wales. Stanner Rock and Hanter Hill, near old Radnor, are largely 

 composed of basic igneous rocks for a description of which we are indebted to 

 Mr. COLE. (I) These rocks vary both in texture and composition. The principal 

 constituents are plagioclase, in one case determined to be labradorite ; augite 

 including diallage ; a rich brown biotite ; opaque iron ores ; a green fibrous 

 alteration product, probably after enstatite ; acicular and compact horn- 

 blende, after augite ; chlorite after hornblende, augite and biotite ; and 

 quartz. Olivine is stated by Mr. COLE to have been very probably a con- 

 stituent of the summit masses of Hanter Hill. 



A fine-grained holocrystalline rock from Stanner, having a specific 

 gravity of 2'86, consists essentially of plagioclase, augite, rich brown mica, 

 titanoferrite and apatite. The mica and augite are often closely intergrown, 

 and both pass over into green alteration products among which hornblende 

 is occasionally recognizable. This rock bears a close relation to the mica- 

 dolerites from the neighbourhood of Saltash, Plymouth, hereafter to be 

 described. As a rule the rocks of Stanner are granular in texture. They 

 are traversed by veins of pegmatite and granophyre, the latter rock fre- 

 quently showing the closest resemblance to the rock which forms the sum- 

 mit of Carrock Fell. 



Mr. WATTS has described a group of intrusive igneous rocks from the 

 Breidden Hills, on the borders of Shropshire and Montgomeryshire, which 

 may be referred to in this connection. They are essentially composed of 

 plagioclase, augite, a rhombic pyroxene and magnetite or ilmenite, together 

 with various alteration products. Two generations of felspar are frequently 

 recognizable, and the structure is often ophitic. The rocks are enstatite- 

 diabases. 



South Wales. Basic igneous rocks are largely developed in the neigh- 

 bourhood of St. David's, Pembrokeshire. They consist of lavas, tuffs, 

 intrusive sheets and dykes. The contemporaneous rocks belong to the 

 Pebidean (pre-Cambrian) of Dr. HICKS, but are referred to the base of the 

 Cambrian by Dr. GEIKIE. The intrusive -rocks are in part of later date. 

 The petrographical characters of these ancient basic rocks have been 

 described by Dr. HiCKS, (2 > Mr. DAVis, (a; Mr. TAWNEY,^) and Dr. GEiKiE/ 5 > 



The basic lavas are described by Dr. GEIKIE as " dull, fine-grained 



(1) The Igneous Rocks of Stanner. G. M., Decade III., Vol. III., p. 219, 1886. 



(2) Q. J. G. S., Vol. XXXIV., p. 147 ; see also Proc. Geol. Ass., Vol. VII., p. 63. 



(3) Q. J. G. S., Vol. XXXIV., p. 166. 



(4) Proc. Bristol Naturalists' Soc., Vol. II., Part 2, p. 109. 



(5) Q. J. G. S., Vol. XXXIX., p. 293. 



