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504 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 3 
contraction. In some conglomerates, for example, the joints traverse 
the enclosed pebbles, as well as the surrounding matrix, m such a 
way that large blocks of hard quartz are cut through by them as_ 
sharply as if they had been sliced in a lapidary’s machine, and the 
same joints can be traced continuously through many yards of the 
rock (Fig. 219). Such instances point to the operation of consider- 
able force.” Further indication of movement is often supplied by the 
rubbed and striated surfaces of joints. These surfaces, termed slecken- 
sides, have evidently been ground against each other. ‘They are 
_ often coated with hematite, calcite, chlorite, or other mineral, which 
has taken a cast of the striz and then seems itself to be striated. 
Joints form natural lines for the passage downward and upward of 
subterranean water. They likewise furnish an effective lodgment for 
the action of frost, which wedges off blocks of rock in the manner 
already described (p. 401). As they serve, in conjunction with 
bedding, to divide stratified rocks into large quadrangular blocks, 
their influence in the weathering of these rocks is seen in the 
symmetrical and architectural as well as splintered, dislocated 
aspects so familiar in the scenery of sandstone and limestone 
districts. 
Occasionally a prismatic or columnar system of joints may be 
observed among stratified rocks, particularly in those which have 
been chemically formed, where, as in the gypsum of the Paris Basin, 
beds are divided from top to bottom into vertical hexagonal prisms.* 
A columnar structure has often been superinduced upon stratified 
rocks (sandstone, shale, coal) by contact with intrusive igneous 
masses (p. 473). 
2. In Massive (Igneous) Rocks.—While in stratified rocks the 
divisiona] planes consist of lines of bedding and of joint, cutting each 
other usually at a high if not a right angle, in massive igneous rocks 
they include joints only; and as these do not as a rule present the 
same parallelism as lines of bedding, unstratified rocks, even though 
as full of joints, have not the regularity of arrangement of stratified 
formations. Some massive rocks indeed may have one system of 
divisional planes so largely developed as to acquire a bedded or 
fissile character. This structure, characteristically shown by phono- 
lites, may also be detected among ancient porphyries (Fig. 220). 
Most massive rocks are traversed by two intersecting sets of chief or 
“master” joints, whereby the rock is divided into long quad- 
rangular, rhomboidal, or even polygonal columns. A third set may 
usually be noticed cutting across the columns and articulating them 
into segments, though generally less continuous and dominant than 
the others (I*ig. 221). When these last-named cross-joints are 
absent or feebly developed, columns many feet in length can be 
1 De la Beche, “ Geol. Observer,” p. 628. 
* See an interesting series of experiments by Daubrée (Comptes Rendus, 1xxxvi. 
1878) on the production of faults and joints; ante, p. 315, 
* Jukes, “ Manual,” 3rd Ed. p. 180. 
