84 
SUBTERRANEAN VOLCANIC ACTION 
BOOK 1 
primarily on the constitution of the parent magma, whence both they and the 
outhowiug lavas have issued. Where the lavas are rhyolites or felsites the 
sills are acid, where basalts have been erupted the sills are basic, though 
there has often been a tendency towards the appearance of more acid 
material, such as trachyte. As wc have seen, considerable differences in 
petrographical characters may arise between the intrusive and extrusive 
offshoots from the same parent magma during the course of a volcanic 
cycle. This cpiestion will be more appropriately discussed together with 
tlie leading characters of Bosses. 
Between the upper and under surface of a thick sill considerable petro- 
graphical variation may sometimes be observed, especially where the rock is 
of basic constitution. Differences both of texture and even to some extent 
of composition can be detected. Sometimes what have been called “ segre- 
gation veins” traverse tlie mass, consisting of the same minerals as the 
General body of the rock, but in larger crystals and in somewhat different 
proportions, d’hat these veins belong to the period of original consolidation 
appears to be shown by the absence of fine-grained, chilled margins, and by 
the way in which the component crystals of the veins are interlocked with 
those of tlie body of the rock. Other veins of finer grain and more acid 
composition probably belong to a later phase of consolidation, when, aftei 
the separation and crystallization of the more basic minerals, the more acid 
mother liquor that remained was, in consequence of terrestrial movements, 
injected into cracks in the now solidified, though still highly heated, rock. 
Examples of these features will be cited from various geological formations 
in the following chapters. 
Pteference has already been made to the difference occasionally perceptible 
between the constitution of the upper and that of the under portions of super- 
ficial lavas. A similar variation is sometimes strongly marked among sills, 
especially those of a basic character, the felspars remaining most abundant 
above, while the olivines and augites preponderate below. Mr. Iddiugs has 
observed some excellent illustrations of this character in the great series of 
sills connected with the volcanic pipe of Electric Peak in the Yellowstone 
country. ^ Some examples of the same structure will subsequently be cited 
from the Carboniferous volcanic series of Central Scotland. 
The greatest extreme of difference which I have observed in the petro- 
graphicar characters of any group of sills is that displayed by the Tertiary 
gabbros of Skye. These rocks occur as sheets interposed among the bedded 
basalts, and injected between each other in such a manner as to form thick 
piles of rudely stratified sills. They possess a remarkable banded structure, 
due to the aggregation of their component minerals in distinct layers, some 
of wdiicli are dark in colour, from the abundance of their iron-ore, pyroxene 
and olivine ; while others are light-coloured, from the predominance of their 
felspar. Erom the manner in which the component minerals of one band 
interlace with those of the contiguous bands, it is quite certain that the 
' “Electric Peak and Sepulchre Mountain,” Vlth Ann. Rep. U.S. Geol. Survey (1890-91), 
p. 584. 
