HORNBLENDE-BASALTS. 
103 
in a specimen (218)* from the top of the 1 300-fb. knoll of Mount Terror it is for the 
most part unaltered. Most of the phenocrysts in this rock, however, show two stages 
of growth, in which variations of chemical composition are indicated by differences of 
optical characters ; a rounded and corroded nucleus of cossyrite-like hornblende, having 
pleochroism from brown (for vibrations across the length) to black (for vibrations 
along the length) is surrounded by the more usual barkevikite-like hornblende with less 
absorption, from bright yellow to deep reddish-brown. The augites in this rock also 
show similar variations in composition, and have generally a pale-green nucleus 
surrounded by a pale-purple zone ; but in this case the change has been continuous, and 
there is no evidence of corrosion of the first-formed green nucleus. 
The hornblende-pseudomorphs in these rocks are similar to those which have 
been often described.f They consist mainly of grains 
and rods of magnetite, sometimes in radiating groups, 
but generally arranged in lines roughly parallel to the 
length of the crystal, with purplish augite and a little 
felspar crystallised about them. With the magnetite is 
usually associated a cossyrite-like hornblende, showing 
pleochroism from deep brownish-red for vibrations 
across the length of fibres to nearly opaque for §5 
vibrations along the length. The pseudomorphs are 
generally surrounded by a narrow, sharply defined 
border of purplish augite like that of the ground-mass. 
According to Becke’s theory f the alteration and 
partial absorption of the hornblende probably took place during phases in the eruption 
in which the pressure diminished much more rapidly than the temperature. 
A quantitative chemical analysis of a hornblende-basalt from the Sulphur Cones 
(385) gave the following result § 
Fig. 59. — Pseudomorph after Horn- 
blende in Basalt (691) from 
Castle Rock, showing inclusions 
of Apatite. (Magnification, 10 diam.) 
SiO, =43-92 
mol. ratios. 
•727 
TiO, = 4-19 
•052 
AL0 3 = 17-42 
•170 
Fe 2 0 3 = 4-09 
•02G 
FeO = 8-83 
•123 
MuO = 0-09 
CaO = 9-53 
•170 
MgO = 4-89 
■121 
Na 2 0 = 4 -GO 
•074 
K 2 0 = 2-17 
•023 
P,0 5 = 0-07 
•005 
H 2 0 at 110° = 0-0G 
H 2 0 above 110° = O'll 
100-57 
* The numbers refer to Mr. Ferrar’s List of Specimens. 
t For bibliography see Hyland, Tsehermak’s Min. Petr. Mitth., 1889, Bd. x, p. 238; also Washington, The 
Volcanoes of the Ivula Basin in Lydia, New York, 1894. 
t Beeke, Tschermak’s Min. Petr. Mitth., 1896-7, Bd. xvi, p. 335. 
§ For discussion of the results of analyses see p. 119. 
