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ine.  GEOGNOSY. 2 4 Bema 
ereenish tints. It will be observed in this, as in other rocks con- 
taining much felspar, that the colour, besides depending on the hue 
of that mineral, is greatly regulated by the nature and stage of 
7 
a 
decomposition. A rock weathering externally with a pale yellow or — 
white crust may be found to be quite dark in the central undecayed 
portion. Besides these differences of aspect arising from varieties of 
colour, ground-mass, &c., distinctions are to be observed according 
to the relative abundance and size of the felspar crystals, and the 
presence of mica (micaceous quartz-porphyry), hornblende (hornblendie 
quartz-porphyry), or other accessory ingredient. When the base is — 
very compact, and the felspar-crystals well defined and of a different 
colour from the base, the rock sometimes takes a good polish, and 
may be used with effect as an ornamental stone. In popular 
language such a stone is classed with the “marbles,” under the 
name of “ porphyry.” 
Closely related to the quartz-porphyries, of which, indeed, it can 
be regarded only as a variety, comes the rock known as elvan or 
elvamte. This is a Cornish term for a crystalline-granular mixture 
of quartz and orthoclase, forming veins which proceed from granite, 
or occur only in its neighbourhood, and are evidently associated 
with it.~ It forms an intermediate stage between granite and 
quartz-porphyry.* 
Felsite (Felstone, Petrosilex), a hard and excessively compact 
flinty-like rock, composed of an intimate mixture of quartz and 
orthoclase. ‘The ground-mass presents under the microscope a 
structure like that of quartz-porphyry, into which felsite naturally — 
passes by the appearance of the porphyritic minerals. 
The quartz-porphyries and felsites occur (1) with plutonic rocks, 
as eruptive bosses or veins, often associated with granite, from which, 
indeed, as above stated, they may be seen to proceed directly; of — 
frequent occurrence also as veins and irregularly intruded masses — 
among highly convoluted rocks, especially when these have been — 
more or less metamorphosed ; (2) in the chimneys of old volcanic 
orifices, forming there the “neck” or plug by which a vent is 
filled up; and (8) as truly volcanic rocks which have been erupted at 
the surface in the form of flows of lava, either (a) submarine, as in 
the felstones of Wales,’ or (b) subaerial, as probably in the quartz- 
porphyry of Arran, and perhaps in the series of “ green-slates 
and porphyries” of the Silurian system in Cumberland,? which 
Professor Ramsay has conjectured to be the products of a subaerial 
volcano. ‘These eruptive rocks are abundant in Britain among forma- 
tions of Lower Silurian, Old Red Sandstone and Lower Carboniferous 
age. In the Inner Hebrides they overlie and alter the Jurassic 
' J. A. Phillips, Q. J. Geol, Soc, xxi, p, 334, Michel-Lévy, Bull. Soc. Géol. France, - 
iii. 3rd ser. p. 201. 
* J. O. Ward, Q. J. Geol. Soc. xxxi. p, 899. The felsite of Aran Mowddwy contains 
83°8 per cent. of silica. 
8 J. O. Ward, op. cit. p. 400, 
