

Parti] JOINTS IN STRATIFIED ROCKS. 501 
Part II.—JoiInts. 
All rocks are traversed more or less distinctly by vertical or highly 
inclined divisional planes termed Joints. Soft rocks indeed, such 
as loose sand and uneompacted clay, do not show these lines; but 
wherever a mass of clay has been subjected to some pressure and 
consolidation, it will usually be found to have acquired them more or 
less distinctly. It is by means of the intersection of joints that rocks 
can be removed in blocks; the art of quarrying consists in taking 
advantage of these natural planes of division. Joints differ in 
character according to the nature of the material which they traverse ; 
those in sedimentary rocks are usually distinct from those in crystal- 
line masses. 
1. In Stratified Rocks.—To the presence of joints some of the 
most familiar features of rock scenery are due (Fig. 217). Joints 


il TEAR 
<i bs — DCAM 
Rie CL ——— at 
a eal 



fl 






b a ia b a 
Fic. 217.—Cuirrs cuT INTO RE-ENTERING ANGLES BY LINES oF Jornt (B.). 
(The faces in shadow (a a) are one set of joints, those in light (6 b) another set). 
vary in the angles at which they cut the planes of bedding, in the 
sharpness of their definition, in the regularity of their perpendicular 
and horizontal course, in their lateral persistence, in number, and in 
the directions of their intersection. As a rule, they are most sharply 
defined in proportion to the fineness of grain of the rock. In 
limestones and close-grained shales, for example, they often occur so 
clean-cut as to be invisible until revealed by fracture or by the slow 
disintegrating effects of the weather. ‘The rock splits up along these 
concealed lines of division whether the agent of demolition be the 
hammer or frost. In coarse-textured rocks, on the other hand, joints 
are apt to show themselves as irregular rents along which the rock 
has been shattered, so that they present an uneven sinuous course, 
branching off in different directions. 
As a rule, they run perpendicular or approximately so to the 
planes of bedding, and descend vertically at not very unequal 
distances, so that the portions of rock between them, when seen 
