CHAP. XXXV 
D YKES OF MORE THAN ONE INFILLING 
161 
connected volcanic episode. In those instances, for example, which have 
been above described, where a central vitreous band has risen along the 
heart of a dyke, the petrographies! affinities of the rocks may be so close as to 
suggest that although the main dyke had consolidated and had subsequently 
been ruptured along its centre by powerful earth-movements, these changes 
all belonged to the same period of dyke-making, and the subsequent uprise 
of glassy material was merely a later phase in the movements of the same 
subterranean magma. 
But where, as probably happens in the large majority of compound 
dykes, there is a strongly marked difference between the respective bands of 
rock, we must either infer that two essentially different magmas co-existed 
in the volcanic reservoirs underneath, and were successively injected into 
the same fissures, or that a sufficient lapse of time occurred to permit a 
total renewal of the nature of the magma, and an uprise of this changed 
material into fissures which sometimes coincided with older dykes. If any 
interlocking of the crystals of the several bands of a compound dyke could 
be detected, we might suppose that the first-injected material had not become 
consolidated and cold before the uprise of the newer rock. But in general 
it would seem that so sharp a line of demarcation can be drawn between the 
two rocks as to indicate that their protrusion was due to two distinct and 
perhaps widely-separated volcanic paroxysms. 
Compound dykes of basic material occur not only among the ordinary 
straight north-westerly series, but also among the less regular gregarious dykes 
and veins, such as abundantly intersect the gabbro bosses. Moreover they 
are to be found among the youngest intrusions, for they traverse the masses 
of granophyre. Conspicuous examples of such late compound dykes are 
displayed along the cliffs of St. Kilda, as will be more particularly described 
in a later Chapter. These St. Kilda dykes often occupy not vertical fissures 
but parallel rents with a gentle inclination (see Figs. 867, 368). 
The Tertiary volcanic series of Scotland furnishes many examples of 
compound dykes of a much more complex character where parallel bands of 
some acid (granophyre, felsite, quartz-porphyry) or intermediate (andesite) 
rock is associated with others of the more usual basic material (dolerite, 
basalt, diabase). As the acid intrusions belong to a comparatively late part 
of the volcanic history, their modes of occurrence will be discussed in 
Chapters xlvi. xlvii. and xlviii. But no account of the general system of 
dykes would be complete without some reference to these compound 
examples, which will therefore be briefly described in the present section 
of this work. 
Early in this century some striking illustrations of the association of 
acid and more basic rocks within the same fissure were noticed by Jameson 
in the island of Arran. ITe described and figured instances at Tormore, on 
the west side of that island, where a group of pitchstones and “ basalts ” or 
andesites have been successively protruded into the same fissures in the 
(probably Permian) red sandstones of that district. 1 
1 Mineralogy of the Scottish Isles, 1800. 
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