CHAP. XXXIV 
THE TER TIA RY D YKES 
ug 
dykes are small and locally numerous ; but it remains singularly charac- 
teristic over the whole region. 
2. The increasing abundance of the dykes as they are traced to the 
west coast and the line of the great Tertiary volcanic plateaux of Antrim 
and the Inner Hebrides. 
3. The rectilinear direction so characteristic of them and so different 
from the tortuous course of local groups of dykes. The exceptions to tin's 
normal feature are as a rule confined to the same localities where depart- 
ures from the prevalent westerly trend occur. 
4. The great breadth of the larger dykes of the system and their 
Fig. 233.— Dyke on the south-east coast of the Island of Mull. 
Persistence for long distances. This is one of their most remarkable and 
distinctive characters. 
5 . The posteriority of the dykes to the rest of the geological structure 
° the regions which they traverse. They are not only younger than the 
other rocks, but younger than nearly all the folds and faults by which 
the rocks are affected. 
6. The manner in which they cut the Jurassic, Cretaceous and older 
ertiary rocks in the districts through which they run. At the south- 
eastern end of the region they rise through the Lias and Oolite forma- 
l °ns, in the west they intersect the Chalk and also the Tertiary volcanic 
p ateaux together with their later eruptive bosses. 
!■ Their petrographical characters, among which perhaps the most 
istmctive is the frequent appearance of the original glass of the plagio- 
e ase-pyroxene-magnetite (olivine) rock, of which they mostly consist. This 
