120 ~ GEOGNOSY AND GEOLOGY. 
called joints. In the actual structure of rocks we must necessarily 
distinguish true cleavage from stratification. 
True cleavage never extends to so great distances as stratification. The 
parts into which cleavage planes divide rocks are referable to a rounded 
and an angular form. Of the former class we have the sphere, the 
spheroid, the ellipsoid, the elliptic spheroid, and the indeterminate surface. 
These forms frequently exhibit a concentric cleavage, as seen in basalt, 
granite, porphyry, &c. (pl. 43, fig. 23). Most frequently, however, the 
cleavage is plane. ; ) 
The angular forms are either indefinite, columnar, or parallelopipedal 
(prismatic). The first class comes nearest to the spherical form. Thus 
lumps of menilite are found in adhesive slate (Jameson) coming very near 
to an aggregation of spherical segments (pl. 43, fig. 22). The columnar 
form is most frequently seen in rocks which have passed from a melted 
liquid condition into the solid. Basalt is especially adapted for the study 
of columnar structures. The six-sided prism must be assumed as the 
primary form from which the three, four, five, seven, &c., have been 
derived as irregular or imperfect developments. In basalt a sphere is not 
unfrequently combined with the column, easily recognised in the alternate 
bending in and out of any two edges of the column, and their accompanying 
thinning and thickening. A single column is generally divided into 
numerous joints by transverse cleavage (pl. 53, fig. 11), the spherically 
convex end of one joint fitting in the spherically concave end of the next 
(pl. 58, fig. 13). When the spherical segment is somewhat greater it 
forms projecting sharp corners to the edges of the prisms, a feature not 
unfrequently seen in the basaltic pillars in the Island of Staffa. Entire 
mountain masses sometimes exhibit this structure, which is especially 
peculiar to lava currents, as shown in pl. 43, fig. 24. The columns often 
consist of small plates inclined irregularly to the principal axis at different 
angles (fig. 25). Porphyritic columns occur in this manner on the 
Wachenberg near Weinheim. A peculiar phenomenon is sometimes 
exhibited when a melted mass, subjected to great pressure, has been forced 
up so as to fill a crack or fissure in the rocks. On cooling, the columnar 
cleavage arises, and the extremities of the columns stand at right angles to 
the sides of the fissure. Thus, if the fissure be vertical, the columns will 
be horizontal, and vice versa. PJ. 48, fig. 16, is intended to elucidate this 
phenomenon. In the vertical fissure, b, the columns are horizontal, while 
the horizontal masses below c and d have vertical columns. In large 
masses this rule does not seem to hold good, the columns being 
combined like billets of wood in a charcoal pit, or else lie grouped 
irregularly, one upon another. This is shown very clearly in the Island of 
Staffa (pl. 49, fig. 7). 
As the columnar form is peculiar to rocks of igneous origin, so the 
parallelopipedal is restricted almost exclusively to those which have been 
deposited from water. Both the rectangular and oblique parallelopipedons 
occur; the former of cubic, pillar, square, and tabular forms. 
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