

JOINTS 151 
contracts while drying, and in this way becomes more or less abundantly 
cracked or fissured. Possibly this may be the origin of many of the 
minor or subordinate irregular joints of sedimentary strata, but it does 
not account for the vertical intersecting joints which are so characteristic 
of these rocks. The passage from the non-crystalline to the crystalline 
condition also involves contraction, and thus we may believe that the 
crystallisation of certain chemically formed deposits may have been the 
cause of their jointed structure. To the same cause must undoubtedly be 
attributed most of the division-planes occurring in crystalline igneous 
rocks, such as the vertical joints in granite, and the prismatic jointing of 
basalt and other eruptive rocks. The cross-joints of granite, however, 
cannot be accounted for in this way. The simple fact that they are 
present only in the upper part of a rock-mass and disappear entirely at 
lower levels suffices to show that they have not the same origin as the 
vertical joints between which they have been developed. It is otherwise 
with the cross-joints of basalt, etc., which appear to be of the same 
nature as the prismatic joints with which they are associated—fissures of 
retreat, due to the contraction of the rock in cooling. The peculiar 
manner in which basalt and many other igneous rocks weather is 
somewhat suggestive. Prismatic columns which have been long 
exposed often lose their angular form, the individual segments or blocks 
assuming a spheroidal shape, so that the rock appears as if built up of 
vertical rows of globular ball-like or cheese-like bodies. Each of these 
spheroids exfoliates in successive concentric shells—a fraction of an inch 
in thickness—and the external ones may be readily detached by the 
hammer. The shells, however, become more adherent and _ less 
conspicuous as we penetrate the rock until they cease to appear. This 
peculiar kind of weathering is not confined to rocks which show a 
prismatic structure, being met with not only in non-columnar basalt but 
in many other igneous rocks, as in some pitchstones, granites, diorites, 
porphyries, etc. The weathering often proceeds so far that the exfoliating 
crusts break down into a kind of earthy or sandy grit, till nothing of 
the original rock may be left save a few scattered balls or cores. It is 
supposed that the shell-like structure betrayed by weathering is really 
original, the centre of each spheroid having been a centre of contraction. 
So long as the rock is fresh the structure remains invisible, and only 
becomes apparent when weathering supervenes. This is a plausible or 
even probable explanation of the phenomena, but it does not quite carry 
conviction. The perlitic structure of glassy rocks (which is due to the 
presence of numerous minute and roughly concentric cracks produced 
during cooling and contraction) has been cited as an example on a small 
scale of that spheroidal structure of basalt, etc., which is only revealed by 
weathering. All one can say is, that nothing comparable to perlitic 
structure has ever been detected by the microscope in those crystalline 
rocks which weather spheroidally—there is nothing in the microscopic 
appearance of basalt, diorite, etc., that would lead one to expect that 
such rocks should exfoliate in successive concentric shells. While, 
therefore, we need have no doubt as to the vertical- and cross-jointing of 
