206 Leonard Hawkes—Tridymite in Icelandic Rocks, — 
rock sections, and a microphotograph showing the characteristic form 
of the tridymite is given in Fig. 1. The crystals are thick and 
frequently show binary twinning after (018). ‘Their resemblance to 
the artificial tridymite from the bricks lining the glass furnaces of 
Appert, Clichy, in France is obvious (ef. Fig. 2, 1, p. 331). The 
commoner form of tridymite, that of thin tables, is well shown in 
a breccia from Faskrudsfjord, East Iceland, consisting chiefly of 
fragments of an acid rock embedded in acid volcanic ash. This rock, 
80 feet in thickness and intercalated in the Tertiary basalts, is 
exposed on the shore about a mile east of the town. The fragments 
of ash and lava contain phenocrysts of soda-microcline and soda- 
pyroxene in (a) a glassy perlitic base and (b) an aggregate of felspar 
quartz and pyroxene grains, respectively. In (a) the perlitic ground- 
mass is often weakly anisotropic, being made up of thin plates of 
tridymite. In (6) similar tridymite occurs together with quartz 
(described below) in cavities, and apparently in the groundmass. 
The nature and mode of occurrence of the two forms of tridymite of 
Hlidarfjall and Faskrudsfjord are in full accord with the theory 
of their mode of origin advanced by Lacroix. 
Quarts after Tridymite. 
The quartz to be described occurs in the rock of Faskrudsfjord. 
The pseudomorphs are of two types, both originating from the thin 
tabular form of tridymite. 
Type I.—Previously noted by Mallard (8, p. 162), Lacroix, and 
Geijer (4). The holocrystalline fragments of the Faskrudstjord 
breccia contain cavities filled with quartz, chlorite, and a small 
amount of a feebly double refracting mineral of low refraction, 
probably tridymite. A microphotograph of a cavity is given in 
Fig. 2. The quartz is of peculiar habit, forming a complex of 
lamellee branching out from one another at different angles. The 
network of laths in the cavity illustrated in Fig. 2 is comprised of 
two individuals, distinguished optically. While several lamellae may 
form one individual of common optical orientation, the laths forking 
without change in optical properties, one straight lath may be 
composed of many individuals. Fig. 3 shows another assemblage of 
laths comprising five individuals. This quartz is undoubtedly 
a paramorph of tridymite, and Geijer has described a similar 
occurrence in cavities in a pre-Cambrian quartz porphyry found as 
boulders in a moraine on Gotland. (See especially the photograph 
on p. 71, 4.) The quartz of the holocrystalline groundmass of the 
Icelandic rock is micropoikilitically developed, and without going 
into the question of micropoikilitic reticulating quartz which has 
been discussed by Geijer, it may be stated that it too, judging from 
its occurrence and association, is very probably a pseudomorph of 
tridymite. It is interesting to note that the soda-microcline pheno- 
erysts, which typically show inclusions of glass, sometimes contain 
lamellar quartz after tridymite (see Fig. 4), thus indirectly supporting 
Lacroix’s theory of the formation of tabular tridymite, ‘‘ de l’attaque 
& haute temperature d’un résidue de matiére vitreuse trés siliceuse.” 
Type II.—The paramorphs of Type I are characterized by the 
