CHAP. VI 
DYKES AND VEINS 
79 
the constituent minerals in, the body of large intrusive masses before con- 
solidation. In particular, the heavier and more basic constituents travel 
towards the cooling margin, leaving the central portions more acid. This 
subject will be more fully considered in connection with the internal constitu- 
tion of Bosses, and some British examples will then bo cited. 
Eeference, however, may here be made to one of the most exhaustive and 
instructive studies of the relations of the subterranean and superficial erupted 
rocks of an old volcano, which will be found in the monograph by Mr. Tddings 
on Electric Peak and Sepulchre Mountain in the Yellowstone Park of 
’Western America. Prom the data there obtainable he draws the deduction 
tliat one parent magma, retaining the same chemical composition, may result 
in the ultimate production of rocks strikingly different from each other in 
structure and mineralogical constitution, yet chemically identical. Electric 
Peak includes the central funnel filled up with coarsely crystalline diorite, 
and having a connected series of sills and dykes of various porphyrites. 
Sepulchre Mountain, separated from its neighbourmg eminence by a fault of 
4000 feet, displays some of the superficial discharges from the vent — coarse 
breccias with andesite-lavas. These rocks are not chemically distinguishable 
from the inti'usive series, but the lavas are, on the whole, more glassy, while 
the materials of the bosses, sills and dykes are more ciystalline. The latter 
display much more visible cpiartz and biotite.^ 
By practice in the field, supplemented by investigation with the aid of 
the microscope, a geologist acquires a power of discriminating wdth fair 
accuracy, even in hand specimens, the superficial from the subterranean 
igneous rocks of an old volcanic district. 
Denudation, while laying bare the underground mechanism of an ancient 
volcano, has not always revealed the evidence of the actual structural rela- 
tions of the rocks, or has first exposed and then destroyed it. Sometimes 
a mass of eruptive rock has been worn down and left in such an isolated 
condition that its connection wdth the rest of the volcanic network cannot 
be determined. So far as its position goes, it might perhaps be either a 
remnant of a lava-stream or the projecting part of some deeper -seated 
protrusion. But its texture and internal structure will often enable a 
confident opinion to be expressed regarding the true relations of such a 
solitary mass. 
i. Dykes and Veins 
For the study of these manifestations of volcanic energy, the British 
Isles may be regarded as a typical region. It was thence that the word 
“ dyke ” passed into geological literature. Thousands of examples of both 
dykes and veins may be seen from the Outer Hebrides southwards across 
the length and breadth of the southern half of Scotland, far into the north 
of England and towards the centre of Ireland. They may be found cut- 
1 12</i. Ann. Hep. U.S. Geol. Survey, 1890-91. As already stated, the eruptions of this volcanic 
centre became progressively more acid, and this change appears to he exhibited by the extrusive 
lavas as well as by the intrusive rocks. 
