274 General Notes. ` [March 
of oligoclase, labradorite, and orthoclase. These crystals, more- 
over, have undergone an unusual alteration into an aggregate of 
colorless prismatic needles of a uniaxial mineral, which occur 
either radially grouped or scattered indiscriminately in the mass 
of their otherwise apparently unaltered host. Their nature 
could not be determined, but an analysis showed that the altera- 
tion is attended with loss of silica and potassium and addition of 
aluminium. The ground-mass of the rock contains numerous 
little plagioclase ‘crystals and grains of epidote, which v. Vogdt 
thinks were derived from the substance of the ground-mass by 
—Certain conglomeratic, granitic, 
Mr. Hicks? thinks are pre-Cambrian in age, have recently been 
described by Bonney.” The so-called felsites from Trefgarn are, 
according to this author, halleflintas of volcanic origin, consist- 
ing of acid lavas and their associated ashes, which have been 
permeated by hot water, containing silica in solution, and have © 
thus been silicified by the replacement of their feldspathic c con- 
stituents by chalcedonic quartz. The greenstones of St. Min- 
ver, Cornwall, have been separated by Rutley3 into two distinct 
varieties. The first embraces those rocks which were once 
position with the production of bands and “small knots” of 
felsitic material, separated by bands of serpentine or palagonite. 
In the felsitic portion are small circular and lenticular areas o 
quartz and serpentine, which the author regards as the fillings 
of original vesicles. The second class described is of much | 
fresher rocks. These contain large areas of augite, polarizing 
as a single individual, in which are included small crystals of 
plagioclase. [This same structure has been described frequently 
y American petrographers* under the term “ lustre-mottling 
(Pumpelly and Irving) and “ poicilitic structure” (Williams).] 
Aggregates of augite, peace ilmenite, and a few accessory 
and secondary minerals make up the entire rock. The author 
calls it an augite-andesite. ser an appendix toan article by Mr. 
Durham 5 on the volcanic rocks of Fife, Professor Judd describes 
altered augite and enstatite andesites, in which the porphyritic 
pyroxene crystals occur in groups, and also porphyritic and per- 
litic mica-dacite glasses. In the base of the latter feldspar micro- 
_ lites and trichites are arranged in flowage lines. When heated 
before the blow-pipe a splinter of this rock lost 8.9 per cent. of 
its weight, and attained a bulk eight or ten times as great as that 
of the original fragment, producing a pumice which readily 
floated on water. The author concludes his paper with a dis- 
. E i Goi. 
5 Quart. Jour. Geol. . Soc., August, 1886, p. 
