THE TERTIARY VOLCANOES 
BOOK VIII 
1 66 
the aid of the microscope. 1 Though most of the instances of such trans- 
formation in Britain occur in the Carboniferous system, and have taken place 
in intrusive rocks of probably, for the most part, Carboniferous or Permian 
age, yet they are not unknown in the Tertiary volcanic series. Some of the 
“ white trap ” of the Coal-measures may indeed belong to the Tertiary period, 
but the coals and carbonaceous shales intexstratified in the Tertiary basalt- 
plateaux have reacted on both the superficial 
lavas and the sills, and have given rise to the 
same kind of alteration as in the Carboniferous 
system, as will be shown in a later Chapter. 
Some marked examples of this alteration 
of intrusive igneous material are to be ob- 
served among the basalt dykes which cut 
the Lower Lias Shales of Skye. These 
shales, where black and carbonaceous, as in 
the island of Pabba, have exercised an un- 
mistakable influence on the abundant dykes 
which intersect them. The chilled selvage 
of each dyke has assumed the dull earthy 
pale-grey or yellowish aspect, which extends 
for a few inches from the wall into the 
Fio. 256. — Dolerite dyke with marginal interior, where it rapidly passes into the 
baims of “white trap,” in black .... , . 
shale, Lower Lias, Pabba. ordinary black crystalline basalt. these 
a, Black carbonaceous Lower i.ias shale ; features will be readily understood from the 
bb, bands of indurated slialc from 15 . . ' 
inches to 2 feet broad ; c, dolerite dyke accompanying diagram (rig. 25 b). Where 
3 feet 3 inches broad; chi, bands of 1 J v ° . ' . 
altered dolerite or “white trap," 3 to o the dykes give oil narrow veins a few inches 
inches broad. ^ 0 . 
broad, these consist entirely ot the £< white 
trap.” The shales are often traversed with strong joints parallel to the 
walls of the dykes, and the transverse joints of the dykes are sometimes 
prolonged into the hands of indurated shale. 
18. RELATION OF DYKES TO THE GEOLOGICAL STRUCTURE OF THE 
DISTRICTS WHICH THEY TRAVERSE. 
In no respect do the Tertiary dykes of Britain stand more distinguished 
from all the other rocks of the country than in their extraordinary inde- 
pendence of geological structure. The successive groups of Palaeozoic and 
Mesozoic strata have been so tilted as to follow each other in approximately 
parallel bands, which run obliquely across the island from south-west to 
north-east. The most important lines of fault take the same general line. 
The contemporaneously included igneous rocks follow, of course, the trend 
of the stratified deposits among which they lie, and even the intrusive sills 
group themselves along the general strike of the whole country. But the 
Tertiary dykes have their own independent direction, to which they adhere 
amid the extremest diversities of geological arrangement. 
1 Tseliermak’s Mineralogische MUtheilungen, ix. (1887), p. 145, and Proc. Roy. Soc. Min. 
