590 
ELEMENTS OF GEOLOGY. 
fissures which often traverse rocks in straight and well-de¬ 
termined lines. They afford to the quarryman, as Sir R. 
Murchison observes, when speaking of the phenomena, as 
exhibited in Shropshire and the neighboring counties, the 
greatest aid in the extraction of blocks of stone; and-, if a 
sufficient number cross each other, the whole mass of rock is 
split into symmetrical blocks. The faces of the joints are for 
the most part smoother and more regular than the surfaces 
of true strata. The joints are straight-cut chinks, sometimes 
slightly open, and often passing, not only through layers of 
successive deposition, but also through balls of limestone or 
other matter which have been formed by concretionary ac¬ 
tion since the original accumulation of the strata. Such 
joints, therefore, must often have resulted from one of the 
last changes superinduced upon sedimentary deposits.^ 
In the annexed diagram (Fig. 626), the fiat-surfaces of rock, 
A, B, C, represent exposed faces of joints, to which the walls 
Fig. 626. 
Stratification, joints, and cleavage. (From Murchison’s Silurian System, p. 245.) 
of other joints, J J, are parallel. S S are the lines of stratifi¬ 
cation ; D D are lines of slaty cleavage, which intersect the 
rock at a considerable angle to the planes of stratification. 
In the Swiss and Savoy Alps, as Mr. Bakewell has re¬ 
marked, enormous masses of limestone are cut through so 
regularly by nearly vertical partings, and these joints are 
often so much more conspicuous than the seams of stratifica¬ 
tion, that an inexjDerienced observer will almost inevitably 
confound them, and suppose the strata to be perpendicular 
in places where in fact they are almost horizontal.]* 
Now such joints are supposed to be analogous to the part¬ 
ings which separate volcanic and plutonic rocks into cuboidal 
and prismatic masses. On a small scale we see clay and 
starch when dry split into similar shapes; this is often 
caused by simple contraction, whether the shrinking be due 
* Silurian System, p. 246. f Introduction to Geology, chap. iv. 
