
JOINTS 145 
however, will be considered in a later chapter, under the 
general head of Lodes. 
As a rule, the more important joints in solidified rocks of all kinds 
tend to be somewhat open, or, at all events, are most readily recognised 
at and for some distance down from the surface, becoming less and less 
conspicuous as they are followed to greater depths, It is impossible to 
doubt that these appearances are due to epigene action, the influence of 
which must gradually die out downwards. The opening of the joints in 
readily soluble rocks like limestone, may be safely attributed to percolat- 
ing water; but, in the case of relatively insoluble rocks the fissures can 
hardly have been opened by the same means, and are more likely to have 
been widened by changes of temperature. In temperate latitudes, 
however, diurnal and seasonal changes of temperature do not affect rocks 
beyond a few feet from the surface, and can scarcely account, therefore, 
for the phenomena referred to. We must remember, however, that 
during relatively recent geological times the present temperate latitudes 
of Europe and North America experienced certain remarkable climatic 
vicissitudes—having sometimes been subjected for lengthy periods to the 
rigours of an arctic climate, while at other times the conditions would 
seem to have been more genial than they are now. Under such alterna- 
tions of cold and heat the rocks could hardly fail to have been affected to 
a much greater depth than is possible at present. We know that in high 
latitudes the ground is permanently frozen to a depth of over a hundred 
feet, and that the heat of summer suffices to thaw only a thin superficial 
stratum. Were glacial conditions, therefore, again to supervene in 
temperate latitudes, we cannot doubt that with increasing cold frost would 
penetrate ever deeper and deeper—the rocks contracting and all moisture 
becoming frozen to depths approximating those reached by frost in sub- 
arcticregions. With the gradual return of genial conditions thawing would 
ensue until no part of the ground remained permanently frozen. To this 
process of alternate freezing and thawing, repeated again and again 
throughout the long glacial cycle, we ought perhaps to assign the open- 
ing up of joints at considerable depths from the earth’s surface. 
Joints in Bedded Rocks.—Sedimentary rocks are usually 
traversed by two sets of joints, perpendicular to the planes of 
bedding, and intersecting each other at approximately right 
angles. Not infrequently these joints run roughly parallel 
for long distances, and when they do so they are known as 
Master-joints. Usually, however, it is impossible to follow 
individual joints very far, and the parallelism of a series is 
only approximate, for often enough one joint runs into 
another. In most cases, indeed, individual joints seem to die 
out in a few yards, and to be succeeded after a longer or 
shorter interval by one or more following the same general 
K 
