JOINTS 149 

columns are vertical. When, on the other hand, the molten 
rock has cooled and solidified in a vertical fissure, the walls 
of this fissure form the cooling-planes, and the columns are 
therefore horizontal. In the case of zztruszve sills or sheets 
of basalt, the columns sometimes extend continuously from 
one cooling-plane to another ; in dyke-like intrusions, however, 
they are usually not continuous, but separated half-way by 
an irregular line. Now and again, indeed, small dykes and 
veins of basalt are wholly composed of successive thin belts 
or layers of prisms—the prismatic layers being separated by a 
series of roughly parallel fissures (see Figs. 73,74, p. 204). In 
lavaform rocks the columnar structure seems likewise to be 
related to planes of cooling—the columns being vertical or 
inclined according as the rock has cooled upon a horizontal 
or an inclined surface. Not infrequently, both in lava- 
form and intrusive rocks, the columns are curved; and in 
most cases, whether curved or straight, they are usually 
intersected at more or less regular intervals by transverse or 
cross-joints, which in some few cases show a ball-and-socket 
arrangement—the convex surface of one segment fitting into 
the concave surface of the next overlying or underlying block. 
For it is to be noted that the convex surfaces of the segments 
in adjacent columns, or even in one and the same column, do 
not always point in the same direction. 
The columns or pillars vary much in size. In some thin 
dykes of basalt they may be less than an inch in diameter 
and only a few inches in length, while in thick sheets and 
lava-flows they may attain a thickness of I or 2 feet, and a 
length of 200 feet or more. Prismatic jointing, although as a 
rule best developed in fine-grained basic igneous rocks, is by 
no means confined to these, for it is often well developed in 
andesites, quartz-porphyries, and now and again in obsidian. 
[ Neither is the structure in question confined to igneous rocks. 
Even sandstone and coal occasionally exhibit a superinduced 
prismatic structure. In such cases the rocks have been in- 
fluenced by the presence of intrusive igneous masses. Excel- 
lent examples occur in the Scottish coalfields, where whole 
beds of coal have been converted into a kind of prismatic 
coke; while at and near their contact with eruptive rock the 
sandstones often acquire a kind of rude columnar structure. |] 
