82 
SUBTERRANEAN VOLCANIC ACTION 
HOOK I 
molten for some time after the sides have consolidated. It the fissure 
served as a channel for the ascent of lava to the surface, it is conceivable 
that the central still fluid part might be driven out and be replaced by 
other material from below, and that this later material might differ con- 
siderably in composition from that which first filled the opening. Such, 
according to Mr. Iddings, has been the probable history of some of the 
dykes at the old volcano of Electric Peak.’- But we can hardly suppose 
that this explanation of compound dykes can have any wide application. It 
could only hold good of liroad fissures having an outlet, and is probably 
inadmissible in the case of the numerous compound dykes not more than 
10 or 15 feet in diameter, where the several bands of rock are sharply 
marked off from each other. The abrupt demarcation of the materials in 
these dykes, their closer texture along their mutual boundaries, the in- 
dications of solution of the older parts of the group by the younger, and of 
injection of the latter into the former, show that they belong to separate and 
unconnected intrusions. These questions will be again referred to in the 
account of the British Tertiary dykes (Chapter xxxv. vol. ii. p. 159). 
Another kind of compound dyke has arisen from the manner in which 
the original fissure has been produced. ‘While, in general, the dislocation 
has taken the form of a single rectilinear rent, which on opening has left 
two clean-cut walls, cases occur where the rupture has followed several 
parallel lines, and the magma on rising into the rents appears as two or 
more vertical sheets or dykes, separated by intervening partitions of the sur- 
rounding rock. Examples of this structure are not infrequent among the 
Tertiary dykes of Scotland. One of these may l)e noticed rising through 
the cliffs of Lewisian gneiss on the east coast of the island of Lewis, 
south of Stornoway. One of the most extraordinary instances of the same 
structure yet observed is that described by I’rofessor A. C. Lawson from the 
Laurentian rocks at the mouth of ‘White Gravel Eiver, on the JI.E. 
coast of Lake Superior. In a breadth of only' about 14 feet no less than 
28 vertically intrusive sheets or dykes of diabase, from 1 inch to 6| 
inches broad, I'i.se through the granite, which is thus split into 27 thin 
sheets. The diabase undoubtedly cuts the granite, some of the sheets 
actually anastomosing and sending veins into the older rock.^ 
From the evidence supplied by the imxlern eruptions of Iceland, it is 
evident that gaping fissures, which are filled by ascending lava and 
thereby converted into dykes, in many instances serve as channels by' 
which molten rock escapes to the surface. It would be interesting if 
any' test could be discovered whereby those dy'kes could be distinguished 
which had ever established a connection with the outer air. It the lava 
continued to ascend in the fissures, and to pour out in sixperficial streams 
for a long time, the rocks on either side would be likely' to undeigo 
considerably more metamorphism than where there was only one rapid 
injection of the magma, which would soon cool. Bossibly in the much 
' 12th Ann. Hep. U.S. (Icol. Siirmj (1890-91), p. 587. 
^ American Oeologist (1894), p. 293. 
