> 
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506 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
its wedges of ice. They likewise give rise to the formation of the | 
fantastic pinnacles and fretted buttresses characteristic of massive 
rocks. 
As lava, erupted to the surface, cools, and passes into the solid 
condition, a contraction of its mass takes place. This diminution 
of bulk is accompanied by the development of divisional planes or 
joints, more especially diverging from the upper and under surfaces, 
and intersecting at irregular distances, so as to divide the rock into 
rude prisms. Occasionally another series of joints, at a right angle to 
these, traverses the mass, parallel with its upper and under surfaces, 
and thus the rock acquires a kind of fissile or bedded appearance. 
The most characteristic structure, however, among volcanic rocks is 
the prismatic, or, as it is incorrectly termed, ‘“‘ basaltic.” Where this 
arrangement occurs, as it does so commonly in basalt, the mass is 
divided into tolerably regular pentagonal, hexagonal, or irregularly 
polygonal prisms or columns, set close together at a right angle to the 
main cooling surfaces (Figs. 222, 223). These prisms vary from 








Fie. 222.—Cotumnar Basaut or Fineau’s Cavz, Starra (Maccuntoen). 
2 or 8 to 18 or more inches in diameter, and range up to 100 or even 
150 feet in length. Many excellent and well-known examples of 
columnar structure are exhibited on the coast-cliffs of the Tertiary 
voleanic region of Antrim and the west of Scotland. In Fig. 222, 
a lower columnar basalt is overlaid by an upper amorphous or non- 
columnar bed. In many cases no sharp line can be drawn be- 
tween such a columnar sheet and the beds above and below, which 
show no similar structure, but into which the prismatic mass seems 
to pass. 
Considerable discussion has arisen as to the mode in which this 
