

148 STRUCTURAL AND FIELD GEOLOGY 
and run into each other so as to divide the rock between 
the vertical joints into a series of lenticular and_ inter- 
osculating layers or sheets. This structure when viewed from 
a little distance sometimes simulates the appearance of 
false-bedding. These curious joints are most conspicuous 
towards the surface, where they are often only a few inches 
apart. The width between them, however, increases with 
the depth, while at the same time the joints become closer 
and more discontinuous, until at last they disappear. The 
joints in question are thus obviously related to the surface, 
and this relation is rendered still more evident by the fact 
that they are always approximately parallel to the surface. 
Thus, when the ground is level the cross-joints are horizontal ; 
but when it is inclined, the joints have a similar dip. Ina 
broad, dome-shaped mountain of granite, for example, the 
rock often appears to consist of a series of rudely concentric 
shells, which at the summit are horizontal, but from thence 
dip outwards in all directions, coinciding roughly with the 
average fall of the ground. 
Cross-joints of the kind described above are not confined to granite, 
but occur in other massive eruptive rocks. They have been observed, 
for example, in some syenites and quartz-porphyries, but are seldom so 
well developed (see Plate XXXVIII.). [Even homogeneous sedimentary 
rocks, such as limestone and freestone, now and again exhibit a similar 
structure, which cannot be mistaken for true bedding. | 
The columnar structure of granitoid rocks due to jointing, 
is by no means so well developed as that of certain other 
igneous rocks. Many basalts, for example, are jointed so 
symmetrically that the rock looks like an organised aggregate 
of prismatic columns (see Plate XXXVII.). When this 
structure is fully developed, as in the well-known rocks of 
Fingal’s Cave and the Giant’s Causeway, the columns tend 
to assume hexagonal forms. But although six-sided columns 
are common enough, yet the faces of the prisms are seldom 
equally developed, while many columns may show fewer or 
more faces than six, so that trigonal, tetragonal, pentagonal, 
and polygonal forms are often associated. This prismatic 
structure is always developed at right angles to the planes of 
cooling. Hence, when the rock is in a horizontal position— 
the upper and under surfaces being planes of cooling—the 
