72 Dr. F. H. Hatch—Soda-Feisites in Wicklow. 
and isolated granules of sphene ; occasionally also a few tiny specks 
of iron-ore. 
The porphyritic structure is not well defined, there being but 
little difference in point of size between the “ porphyritic” felspar 
and the crystals of that mineral in the more evenly crystalline 
portions of the ground-mass. It is only the occurrence of an 
isolated large crystal in a eryptocrystalline part that gives the 
porphyritic appearance. These large felspars form broad rect- 
angular crystals, which are sometimes slightly rounded. In some 
cases they present a fine twin-lineation either on one (the albite) or 
two (albite and pericline) types. In other cases the crystals show 
no trace of twinning. Such crystals, however, are characterized by 
another and somewhat remarkable structure. They appear, namely, 
between crossed Nicols to be divided up into a number of rect- 
angular patches by narrow partitions, which have an extinction- 
angle differing slightly from that of the main portion. In other 
cases the central portion extinguishes uniformly, while a marginal 
layer goes out at a slightly different angle. The latter case can 
sometimes be made out without the use of the Nicols; the rim of 
the felspar-section being clear, while the central portion is speckled 
over with minute opaque particles, giving it a somewhat cloudy 
appearance. Have these phenomena, more especially the first- 
mentioned, anything in common with the “felderweise mikroper- 
thitische”” structure mentioned by Lossen and Rosenbusch as 
characteristic for the felspar of the keratophyres? The smaller 
felspars of the ground-mass are mostly striated. 
A chemical analysis of this rock gave me the following result :— 
$10. 500 | 000 = 77°29 
IGOR G05) Ss ‘ 
Fe03 (a trace) be ais Ta62 
CaO (a ae 30.000 Bag! do cao, Odd — 
MgO ... RAED) 3G) \) CORREA EH Bets wk eA y Ee 38 
K 50 — "16 
Na,0 Lares gc = (oo 
Loss on ignition ... = iT 
100°62 
Sp. G. = 2°64 
From this analysis. the mineral composition of the rock was 
calculated to be the followings — 
JAS) CPUENAWA (465) 440°." sod’ Gageubeea) dos) ona. SS BAD) 
Orthoclasesi ea) teh ae ce 95 
Albite... aH = 64°33 J 
Felspar...  ... o54. 908) 1000 | con) O80 65°28 
Other substances .. JA CeIMOAC coo Waodin cae eh As} 
100:00 
This analysis shows that the rock consists almost entirely of 
quartz and a soda-felspar (albite). 
During two visits to Counties Wicklow and Waterford in June 
and November of last year I collected, among other rocks, a large 
series of felsites. It is exceedingly probable that a petrographical 
