CHAP. XXXV 
CONTACT-METAMORPHISM OF DYKES 
163 
In this instance, and generally throughout the district, there is nothing 
to indicate that the different bands of the dyke have any relation to each 
other as connected uprises of material from the same original magma which 
was either heterogeneous or was undergoing a process of differentiation 
beneath the terrestrial crust. On the contrary, the several parts of each 
dyke are as distinctly marked off from each other as they could have been 
had they been injected at widely separated intervals of volcanic activity. 
Mr. Harker, in the course of his survey of this Skye ground, has observed 
that “ where evidence is available, the central acid dyke is found to be newer 
than the basic ones. It has not split a single basic dyke, but has insinuated 
itself between the two members of a double dyke. This is more clearly seen 
when the acid magma has been forced into a triple or multiple basic dyke ; 
the perfect symmetry of arrangement may in this case be lost. Tor instance, 
on the shoie north-east of Gorry, Broadlord, a 13 ieet dyke of granopliyre 
occurs in a multiple dyke of basalt, but it has taken its line so as to leave 
only a one-foot dyke on one side, and a group with a total width of 12 feet 
on the other. Also it has not accurately kept its course, but has cut 
obliquely across one of the group of dykes alluded to. I11 some cases it is 
certain that the acid magma has to some extent dissolved a portion of the 
wall of a basic dyke with which it has come in contact. This may account 
tor the magma finding its easiest path along, and especially between, pre- 
existing more basic dykes.” This subject will be again referred to in Chapter 
xlviii., when the phenomena of compound sills are discussed. 
Before closing this account of compound dykes, I may remark that 110 
examples have yet been observed among the ordinary Tertiary dykes of 
Britain where, by a process of differentiation between the walls of a fissure, 
successive zones have been developed in the dyke, differing from each other 
m structure and composition, but becoming progressively and insensibly 
more acid towards the centre, such as have been described from the older 
i-ocks of Norway and Canada. Among the Tertiary gabbro bosses, indeed, 
there occur sheets or dykes which present a remarkably handed structure! 
to which full reference will be made in later pages. But I have never seen 
anything at all resembling such a structure among the dykes of andesite, 
dolerite, or basalt. 
17 . CONTACT-METAMORPHISM OF THE DYKES 
A geologist might naturally expect that such abundant intrusions of 
Igneous rock as those of the dykes should be accompanied with plentiful 
Proofs of contact -metamorphism. But in actual fact, evidence of any 
serious amount of alteration is singularly scarce. A slight induration of the 
rocks on either side of a dyke is generally all the change that can be 
detected. 
Some of the larger dykes, however, show more marked metamorphism, 
le nature of which appears in many cases to be chiefly determined by the 
c mnncal composition of the rock affected. Thus a considerable alteration 
