THE TERTIARY VOLCANOES 
BOOK VIII 
1 86 
along these picturesque shores. Successive sheets of lava, either horizontal 
or only gently inclined, rise above each other from base to summit of the 
cliffs as parallel bars of brown rock with intervening strips of bright green 
grassy slope. 
The geologist who for the first time visits these coast-lines is impressed 
by the persistence of the same lithological characters giving rise to the same 
topographical features. He soon realises that the plateaux, so imposingly 
truncated by the great escarpments that spring from the edge of the sea, 
are built up essentially of dark lavas — basalts and dolerites — and that 
fragmental volcanic accompaniments, though here and there well developed, 
play, on the whole, a quite insignificant part in the structure and com- 
position of these thick piles of volcanic material. Closer examination 
in the field enables him to ascertain that, regarded as rock-masses, the lavas 
include four distinct types : — 
1st. Thick, massive, prismatic or rudely-jointed sheets, rather- more 
coarsely crystalline and obviously more durable than the other types, inas- 
much as they project in tabular ledges and tend to retain perpendicular 
faces owing to the falling away of slices of the rock along lines of vertical 
joints. Many rocks of this type are undoubtedly intrusive sheets, and as 
such will be further referred to in a later chapter. But the type includes 
also true superficial lavas which show the characteristic slaggy or vesicular 
bands at their upper and lower surfaces. The mere presence of such bands 
may not be enough, indeed, absolutely to establish that the rock possessing 
them flowed at the surface as a lava, for they are occasionally, though it 
must be confessed rarely, exhibited by true sills. But the rough scoriaceous 
top of a lava-stream, and the presence of fragments of this surface in the 
overlying tuff, or wrapped round by the next succeeding lava, sufficiently 
attest the true superficial outflow of the mass. 
2nd. Prismatic or columnar basalts, which, as at the Giant’s Causeway 
and Staffa, have long attracted notice as one of the most striking topogra- 
phical elements of the plateaux. Columnar structures are typical of the 
more compact heavy basalts. A considerable variety is observable in the 
degree of perfection of their development. Where they are least definite, 
the rock is traversed by vertical joints, somewhat more regular and close-set 
than those in the dolerites, by the intersection of which it is separated into 
rude quadrangular or polygonal columns. The true columnar structure is 
shown in two chief forms. ( a ) The rock is divided into close-fitting parallel, 
usually six-sided columns ; the number of sides varying, however, from three 
up to nine. The columns run the whole thickness of the bed, and vary 
from 8 or 10 to 40 or even 80 feet in length. They are segmented 
by cross joints which sometimes, as at Giant’s Causeway, take the ball-and- 
socket form. Occasionally they are curved, as at the well-known Clam-shell 
cave of Staffa. (6) The prisms are much smaller, and diverge in wavy 
groups crowded confusedly over each other, but with a general tendency up- 
wards. This starch-like aggregation may be observed superposed directly 
upon the more regular columnar form as at the Giant’s Causeway and also 
