806 THE JOURNAL OF NGELOLOG NE 
Thin sections of the rock show a characteristic ophitic struc- 
ture, with in places a flow structure around the amygdules brought 
out by the feldspar microlites. The angular interstices between 
the well preserved lath-shaped crystals are filled by irregular 
pieces of fibrous green hornblende, with here and there epidote, 
zoisite, less chlorite, and grains of titanite, the result of the alter- 
ation of the original augite and titano-magnetite, and also of any 
original vitreous base which may have been present. The pres- 
ence of a base is indicated by the presence of chloritic material 
between some of the feldspars, seemingly the alteration product 
of the base in which the crystals were imbedded. Moreover the 
sharply bounded walls of the amygdules may indicate the former 
presence of glass.* Olivine has in no case been observed. The 
feldspar is very fresh, only beginning to become slightly turbid 
through development in it of a few grains of epidote and flakes 
of chlorite. Rarely does any original magnetite remain, and in 
such cases the rock is very noticeably darker. 
Apoandesite.—The apoandesites include those rocks which, 
while intimately associated with and very closely related to the 
apobasalts, differ from them in their porphyritic habit, having 
labradorite as phenocrysts. 
In thin section the large automorphic phenocrysts of slightly 
altered plagioclase lie in a very fine grained matrix, composed of 
long slender feldspar microlites separated by crystals of magnet- 
ite, though in most cases it has entirely disappeared,—and 
the alteration products of the original augite and base, fibrous 
green hornblende, epidote, zoisite, chlorite, and brownish grains 
of a mineral which has a higher single and double refraction 
than the epidote, and is presumed to be titanite. Measurements 
made against the twinning plane of the plagioclase phenocrysts 
give an average extinction angle of 25°, indicating the feldspar 
to be labradorite. 
The character of the microlites I have not been able to deter- 
mine with sufficient accuracy. The alteration which most of 
them have undergone makes the task of searching for suitably 
tJ. J. SEDERHOLM: Studien tiber archzeische Eruptivgesteine aus dem siidwest- 
lichen Finnland. TYTsch. Mit. XII., p. 113. 1891. 
