
JOINTS 153 
minerals, as in the case of phonolite, which, owing to the orientation of 
its dominant minerals, often cleaves more or less readily into parallel 
slabs and flags. So again, many lavaform rocks, and even occasionally 
intrusive sheets or sills and dykes, have a tendency to split most readily 
in the direction of flow. In most cases the “grain” is determined by the 
more or less obvious parallel arrangement of the rock-ingredients—a 
rock dividing most readily when the orientation is best developed. 
When such arrangement is very obscure or not at all visible, as in heavy 
lavas and sills, the coincidence of the “grain” with the direction of flow 
seems, nevertheless, to show that the “grain” is an original structure. 
No such structures, however, occur in normal granite—the grain of which 
certainly does not depend on the alignment of its constituents. Neither 
does it bear any relation to possible movements of the original molten 
mass, nor to the vertical joints or fissures of retreat produced by con- 
traction. It has apparently been superinduced in granite by epigene 
action—and is not improbably due to the expansion and contraction 
induced by seasonal and secular changes of temperature. This conclusion 
gains support from the fact that cross-jointing and coincident “ grain,” 
similar to those of granite, have been observed in other kinds of eruptive 
rock. Cross-jointing has even been superinduced in the upper parts of 
fine-grained homogeneous aqueous rocks, where consolidation from a 
state of igneous fusion is, of course, excluded. But if such structures 
can be superinduced by epigene action, we may be led to suspect that the 
exfoliating spheroids of weathered basalt, etc., are likewise independent 
of any original structure due to contraction while the rock was cooling— 
that, in short, the concentric shells are solely the result of changes of 
temperature and of weathering generally. 
CRUSTAL MOVEMENTS.—While it may be admitted that the tension 
brought about by the causes already considered must have induced the 
formation of fissures in many kinds of rock, there is yet a large class of 
joints which cannot be thus explained. The regular intersecting systems 
of master-joints in sedimentary strata are suggestive rather of powerful 
mechanical stress and strain. The strata have been cut through as 
smoothly as if they had been severed by a knife ; and this is true not only 
of homogeneous rocks, such as limestone, but of heterogeneous aggregates 
like conglomerate. In the latter rock the superinduced division-planes 
pass without interruption through stones and matrix alike, even although 
the stones may consist of much harder and tougher material than the 
matrix in which they are embedded. Had such joints been the result 
of contraction only, the stones would simply have been pulled out of the 
matrix on one side of a fissure and left projecting from the surface of the 
other. The general opinion is that joints of this kind are the result of 
crustal movements. It is not difficult to conceive how strata, subject 
to compression and tension during such movements, should have cracked 
and become fissured. Weseem thus to get a ready explanation of the 
strike-joints, for it is along the axes of folds that strata would be subject 
to the greatest compression and tension. It seems also reasonable to 
infer that dip-joints might have originated at the same time. Various 
