548 Dr. F. H. Hatch—L. Silurian Felsites of S.E. Ireland. 
appearance produced by the uniform distribution of minute prisms of felspar among 
the rounder granules of quartz. ‘The porphyritic felspar, which is not abundant, 
occurs in broad rather irregular grains, presenting sporadic twin-striation. 
Scales, shreds and streaks of both muscovite and chlorite are rather abundant. 
10. (I. 80.)—Half-mile north-west of Woodenbridge, on the road from Wooden- 
bridge to Aughrim, Co. Wicklow. Sheet 139. 
A grey almost phanerocrystalline felsite, like No. 9. 
Under the microscope: Microcrystalline ground-mass, with porphyritic grains of 
quartz and felspar, the former much corroded and rounded, the latter showing 
occasional twin-striation. 
11. (I. 114.)—Ridge, immediately south-east of cairn on Castletimon Hill, Co. 
Wicklow. Sheet 130. 
Compact felsite, of light-grey colour, but with dark mottlings. 
The micro-structure of this rock is extremely varied: at one place cryptocrystal- 
line, presenting the characteristic dappled ‘‘felsitic’’ appearance between crossed 
Nicols; at another, almost coarsely microcrystalline, the grains of felspar and 
quartz being clearly distinguishable. The structure is, however, nowhere truly 
porphyritic. 
12. (I. 96 )—Old Quarry, Little Rock, Arklow, Co. Wicklow. Sheet 139. 
Light-coloured felsite, spotted white with porphyritic crystals of felspar. 
Under the mieroscope this rock is seen to consist of fairly large porphyritic crystals 
of quartz and felspar embedded in a microcrystalline ground-mass. The latter is 
made up of a uniform minutely granular aggregate of quartz and felspar, together 
with minute flakes and scales of muscovite and chlorite. 
The quartz occurs in round and corroded grains, of which two or three are often 
closely aggregated. The felspar is present in broad rectangular crystals, presenting 
good crystallographic contours. The crystals occasionally show dual twinning ; more 
frequently, however, the twinning is polysynthetic, the crystals being striated either 
in one direction alone (albite type) or in two directions at right angles to one another 
(albite and perieline types). A slight development of zonal structure is here and 
there to be observed. In no case was the extinction-angle found to exceed 20°. 
Group I. comprises felsites containing few or no porphyritic 
crystals; they are composed mainly of a truly eryptocrystalline or 
felsitic aggregate. The few porphyritic crystals are not striated, 
and consist doubtless of orthoclase. 
Groups II. and III. embrace felsites in which a striated porphyritic 
constituent is more or less abundant. On the other hand, the felspar 
of the ground-mass can be safely referred to orthoclase, as in Group I. 
And it seems likely that the fluctuation in the percentage of potash 
and soda in these rocks may be due to a variation in the relative 
proportion between a porphyritic albite-felspar and the orthoclase- 
felspar of the ground-mass. It is not impossible, however, that 
the porphyritic felspars may belong to a triclinic potash-soda series 
(the anorthoclase series of Rosenbusch'), analogous to the soda- 
orthoclase or soda-microcline described by Forstner ? in the liparites 
of Pantellaria, by Brogger® in the augite-syenite of Southern 
Norway, and by Miers and Fletcher* from Kilimanjaro. In all these 
cases, however, the analyses show the presence of 2 or 3 per cent. of 
lime, which, with the notable exception of the Little Arklow Rock 
(No. 12), is almost entirely absent from the Irish soda-felsites. 
Unfortunately the felspar-grains in the latter are too small for 
1 Mikros. Physiog. vol. i. (1885), p. 550. 
® Zeitsch. f. Kryst. vol. viii. (1884), p. 125. 
8 Die Silurischen Etagen 2 and 3, 1882, p. 261. 
4 Min. Mag. vol. vii. (1887), p. 181. 
