


152 STRUCTURAL AND FIELD GEOLOGY 
basalt, etc., being due to contraction, the origin of the shell-like 
structure exhibited by weathering and decomposing rocks is still an open 
question. 
EXPANSION.—Rocks of all kinds when subjected to heat will neces- 
sarily expand, and when cooling will contract. Asa result, they become 
rent and fissured. Excellent examples are seen in the Scottish coal- 
fields. Thus beds of common coal, subjected to the heat of molten rock 
erupted in their immediate neighbourhood, have been converted into 
prismatic coke. In such cases the coal having been subjected to 
destructive distillation has, of course, lost some of its constituents. Even 
siliceous sandstones and argillaceous shales invaded by eruptive rock- 
masses often acquire a rudely columnar structure. But sun-heat is a 
much more general cause of expansion. This is probably effective in all 
latitudes, but naturally enough its results are best studied in dry tropical 
and subtropical regions. In temperate and higher latitudes, the effect of 
insolation is obscured or entirely concealed by the much more energetic 
action of frost and other epigene agents. In warm and relatively rainless 
regions the rocks are heated up during the day toa high temperature— 
consequently their superficial portions expand to such an extent that 
they often become detached, and bulge up from the underlying rock of 
which they form a part. In this way igneous rocks sometimes acquire 
a superficial flaggy structure. When night falls rapid radiation ensues, 
and the rocks quickly contract, so that the superficial portions tend to 
break up more or less rapidly. In the case of fine-grained homogeneous 
rocks, the highly heated surface often peals away in thin sheets which curl 
up, and are readily removed by the wind. The flags produced by desquama- 
tion naturally coincide with the surface, so that they may be curved, 
inclined, or horizontal according as the rock-surface is rounded, sloping, 
or level. Their direction is, therefore, independent of any internal rock- 
structure. 
The cross-joints of granite may, in like manner, owe their origin 
to epigene action. All the phenomena connected with them seem to 
point to that conclusion. They are always approximately parallel with 
the surface, are most numerous and strongly marked in the superficial 
part of the rock, and as they are followed downwards, appear at longer 
and longer intervals, becoming, at the same time, more interrupted in 
their course and less conspicuous, until finally they disappear. Further, 
it may be noted that the “grain” or “rift” of the granite—z.e. the direction 
in which the rock splits or breaks most readily, is parallel with the cross- 
joints. Not only so, but, like the latter, it is most marked near the 
surface, and gradually dies out downwards as they disappear. Now there 
is no apparent petrographical structure to account for this “grain,” and 
its coincidence with the cross-jointing. The rock consists throughout of 
a heterogeneous pell-mell aggregate of minerals. It is otherwise with 
sedimentary rocks, the “grain” or “rift” of which naturally coincides 
with planes of deposition, just as the ‘‘ grain” of a schistose rock coincides 
usually with planes of foliation. The grain of some crystalline igneous 
rocks is due, likewise, to a roughly parallel arrangement of their constituent 
