174 
THE TERTIARY VOLCANOES 
BOOK VIII 
streaks which mark the course of basalt-dykes through that rock. More- 
over the greater liability of the material of the dykes to decay causes them to 
weather into long lines of notch or recess. Four or five such dykes follow 
each other in nearly parallel bands, which slant upward from the sea-level 
on the eastern face of the hill Conacher to a height of several hundred 
feet. 1 (Fig. 258, see also Fig. 367.) 
Ihe acid eruptions of the Inner Hebrides are marked by so varied 
a series of rocks, and so complex a geological structure, that they may, 
with some confidence, be regarded as having occupied a considerable 
interval of geological time. Yet we find that this prolonged episode in the 
volcanic history was both preceded and followed by the extravasation of 
basic dykes. 
Eeference has already been made to recent observations by Mr. Harker, 
who, in mapping the Strath district of Skye for the Geological Survey, has 
not only confirmed the generalization as to the existence of a series of dykes 
earlier, and another later, than the great granophyre protrusions of the Inner 
Hebrides, but has made some progress towards the detection of a means of 
distinguishing the two series even where no direct test of their relative age 
may be available. He thinks that the general habit and petrographical 
characters of the dykes may on further investigation be found to afford a 
sufficiently reliable basis for discrimination. He finds that where the 
relative ages of the dykes with reference to the granophyre can be fixed, 
the earlier or pre-granophyre series is without exception basic. It consists 
of fine-textured basalts or diabases, without any conspicuous porphyritic 
crystals. Its dykes are less regular and persistent in their bearing than 
those of the later series ; have frequently a considerable hade, even as much as 
45 J , and often show chilled edges with tachylitic selvages. In Skye many of 
these earlier dykes may be connected with the gabbro. They appear to be 
more basic and to have a higher specific gravity than those of the later 
series which most resemble them. 
The later or post-granophyre dykes include several types, the relative 
ages of jvhich are not yet definitely fixed. They run in straight parallel 
lines, and thus seldom intersect each other. They are generally vertical or 
highly inclined, and are much more frequently characterized by amygdal- 
oidal structure than the earlier series. Mr. Harker distinguishes the 
following varieties among them : ( a ) Quartz-felsites and other acid rocks ; 
these are not very common. (6) Pitehstones and various spherulitic and 
variolitic rocks : the actual pitehstones observed are comparatively few 
in number, but it is certain that some of spherulitic varieties are devitrified 
pitehstones. (c) Basic rocks, not conspicuously porphyritic and less de- 
cidedly basic than the dykes of the pre-granophyre series ; most of the later 
groups come into this or the next group, (d) Porphyritic basic dykes 
not infrequently carrying inclusions of gabbro, granophyre or other rocks. 
The porphyritic felspars seem to be in great part of foreign derivation, and 
1 This relation of the later dykes to the granophyre was observed here by Maoenlloeh ( Western 
Isles, vol. ii. p. 55). 
