Dr. F. H. Hatch—Soda- Felsites in Wicklow. al 
frequently accompanied by felspathic tuffs; and there is no doubt 
that they are the product of contemporaneous extrusion during the 
deposition of the sedimentary rocks in which they occur. 
Microscopical examination and chemical analysis show that these 
rocks consist, in part at least, of soda-felsites or keratophyres. The 
keratophyres (so named from their resemblance to hornstone*) were 
first described by Giimbel;* but it is to K. A. Lossen® that we are 
mainly indebted for the investigation of the characters of these 
peculiar rocks and the vindication of their claim to consideration 
as a definite rock-type. They are characterized by a remarkably 
high percentage of felspar, especially of soda-felspar, which in 
some cases appears to be soda-orthoclase or soda-microcline, in 
others, albite. In consequence, splinters of the rocks fuse more 
readily before the blow-pipe than the normal quartz-orthoclase- 
felsites. 
Normally constituted felsite, that is to say, rocks composed of a 
eryptocrystalline aggregate of quartz and felspar (in great part 
orthoclase), with or without porphyritic quartz, also occur in the 
Wicklow District;4 but there is little doubt that the two types 
graduate into one another. 
Some specimens, collected from a rocky eminence a quarter of a 
mile west of Brittas Bridge, seven and a half miles west of Rath- 
drum (Sheet 130), will serve to illustrate the characters of the 
keratophyres of this district. The rock cropping out at this place 
is a compact felsite, having, when weathered, a curious mottled 
appearance, due to the presence of numerous greyish-brown spots 
of about a quarter of an inch diameter. These spots are crowded 
closely together, being only separated by a small quantity of a dark 
grey interstitial substance. Where still more exposed to the action 
of the weather, the rock is coated with an opaque white crust. 
When fresh it is of a dark bluish gray to black colour and splinters 
readily under the hammer. 
Under the microscope it is seen to be composed almost entirely of 
felspar and quartz, the former being in excess. The few porphyritic 
crystals are invariably felspar, the quartz being confined entirely to 
the ground-mass. The texture of the latter is extremely variable. 
In general it is microcrystalline, consisting then of square and 
lath-shaped sections of felspar, between which are entangled 
irregular grains of quartz; in places, however, the texture sinks 
almost to cryptocrystalline, the individual granules becoming so 
small as to be scarcely distinguishable ; but even then, by the use of 
a higher power, the microcrystalline structure can generally be 
made out, and there is no evidence of the presence of any ‘‘ micro- 
felsitic ” (isotropic) matter. 
Scattered sparingly through the sections are scales of chlorite, 
1 Gr. xépas, a horn. 
2 Die palaolithischen Eruptivgesteine des Fichtelgebirges, Munich, 1874, p. 45. 
3 Zeitsch. deutsch. Geol. Ges. xxxiv. (1882), pp. 199 and 445; xxxv. (1883), p. 215; 
Jahrb. k. preuss. Geol. Landesanst. fiir 1884 (1885), p. 21. 
4 See Explan. Mem. on Sheets 138 and 139 of the Map of the Geol. Survey of 
Ireland, 1888, p. 0. 
