Vol. 65.] ORDOVICIAN AND SILURIAN EOCKS OF CONWAY. 191 



very similar to No. 1 ; the phenocrysts are fewer in number, one 

 group showing, however, a radial arrangement, and the trachytic 

 character of the ground-mass is more pronounced. 



Slide III. Green rhyolite, Railway-bridge. — In this rock the 

 felspar-phenocrysts are larger and more conspicuous than in any 

 other rock of the series ; they are mainly orthoclase, some still show 

 Carlsbad twinning, while others are almost entirely replaced by 

 calcite or dolomite. The ground-mass is distinctly micropcecilitic in 

 places, the trachytic character being less pronounced, and the lines of 

 flow are well marked by bands of opaque dust. There is also some 

 ilmenite present, as well as a chloritic mineral of secondary origin. 



Slide IV. Dark- grey rhyolite, top of hill above Reservoir. — 

 This rock is the most characteristic of the ' trachytic ' types : the 

 felspar-phenocrysts are mainly oligoclase, as are also the microlites 

 of the ground-mass ; flow-banding is beautifully shown, and micro- 

 pcecilitic structure clearly indicated. 



All these four rhyolites are members of the Lower Banded Group. 



Slide V. Lower Creamy Rhyolite, Road-quarry II. — The 

 phenocrysts in this rock are of oligoclase and pseudomorphs after 

 an augite of somewhat peculiar habit. The ground-mass varies : 

 some bands show minute felspar- microlites, others are micropcecilitic, 

 and there is much to support the hypothesis that the rock was once 



Slide VI. Upper Creamy Rhyolite, western end of Conway 

 Mountain. — There are in this rock a few highly altered phenocrysts 

 of felspar, in a conspicuously micropcecilitic ground -mass. 



Slide VII. Upper Creamy Rhyolite, near the ' tear '-fault, 

 eastern end of Conway Mountain. — This slice shows the effect of 

 shearing, sericite being developed along more or less definite 

 planes of foliation. 



Slide VIII. Penmaenbach rock, Headland Quarry, Penmaen- 

 mawr Road. — This is the rock which occupies the whole of the hill of 

 Penmaenbach. It is a light-grey rhyolitic rock, which is, however, 

 rather more coarsely crystalline than any of the lavas on Conway 

 Mountain, and appears to fill an old neck. Under the microscope 

 it shows some highly altered crystals of felspar and (?) bronzite in a 

 micropcecilitic ground-mass, the felspar-microlites and the enclosing 

 quartz-grains being of larger size than in the common type of 

 rhyolites of Conway Mountain. 



A section of this rock taken from the edge of the neck shows that 

 the rock is more conspicuously porphyritic there, and appears to be 

 also richer in quartz. 



