458 Th. Thoroddsen—Voleanie History of Iceland. 
from the preceding, under Gyrn Goch near Clynnog. The hand- 
specimen is rather coarsely crystalline; a dark-green ground with 
bronze-coloured bisilicates. Golden-coloured mica fairly abundant. 
Active effervescence with acid. 
Microscope.—The predominant mineral in the slide is hornblende. 
Of this, there appear to be three varieties: (1) Seemingly an 
original constituent with characteristic cleavage, and strong dichroism, 
giving shades of brown; (2) perhaps an altered form of this, less 
strongly dichroic, giving shades of green; (8) certainly of secondary 
formation, a kind of tremolite, almost colourless. There is, however, 
some augite, also an original constituent. There appears to have 
been a fair amount of felspar, but it is now replaced by various 
secondary constituents, such as earthy matter, zeolites, calcites, etc. 
Opacite, perhaps magnetite, a little apatite, calcite, serpentinous 
looking, and other decomposition products are present. 
[Cl. 1.] Pentrebach, a hill BH. of Llanaelhairn near Clynnog; a 
purplish-grey, fine-grained felsitic rock with small brownish-yellow 
felspars; slight effervescence with acid. 
Microscope.—The ground-mass is rather decomposed, but shows 
in places the remains of a wavy branching fluidal structure. With 
crossed Nicols we see it to be cryptocrystalline. There are some 
grains of quartz and a good many crystals of rather decomposed 
felspar; plagioclase seemingly predominating. “There is a fair 
amount of iron peroxide, probably hematite, in most cases associated 
with and replacing parts of felspar crystals. We note also a little 
epidote and traces of a pyroxenic mineral. There is a curious case 
of one quartz crystal inclosing another. The rock was probably 
once a lava, akin to rhyolite. Ws Gabe 
Ty.—An Account oF Votcanic Eruptions AND HARTHQUAKES 
WHICH HAVE TAKEN PLACE IN ICELAND WITHIN Historica ‘TrIMEs. 
By THoryvaLtpr THORODDSEN. 
4 EW spots on the face of the globe of the extent of Iceland could 
be mentioned where, within the same space, an equal number 
of volcanos could be found. In that island, too, many volcanic 
eruptions have occurred, which have been, perhaps, of greater 
magnitude than any others recorded as having taken place any- 
where else on the earth. And yet the volcanos of Iceland are but 
imperfectly known, and, with the exception of two or three, not one 
of the rest has ever been scientifically examined. Hence the fact, 
that geological manuals so frequently contain most erroneous state- 
ments concerning this subject. A general uncertainty prevails as to 
how many volcanos have actually been in a state of eruption within 
historical times; dates are dealt with in a confused manner; and 
frequently mountains come to figure as voleanos which have never 
shown any signs of eruptive activity. 
_ Yet, by critically sifting Icelandic records on this subject existing 
in print and MSS., a pretty exact and accurate list may be drawn 
