BASALTIC AND ALLIED ROCKS. 507 
The faults and curvatures in a few dikes are represented in the fol- 
lowing figures. The faults in some instances take place every few 
Pe == See ae 
—— 
Fig. 4. ee ee ee 
feet, and vary in amount from a few inches to some yards. ‘These 
faults show that the enclosing rock has undergone greater distur- 
bances than would be inferred from other appearances. 
These dislocations are more striking in dikes intersecting the basalt, 
which is the fact with those represented in figures 2 to 5; and it evi- 
dently arises from the tendency of the basalt to fracture irregularly in 
all directions, instead of in long straight lines, like the sandstone. 
Prismatic and Concentric Structure in the Basalt.—The regularity 
and magnitude of the basaltic columns in some portions of Illawarra 
have been alluded to on a former page, and a figure of the Kiama 
Causeway has been given. 
The columns of Kiama are surprisingly perfect, and vary from 
five to eight feet in diameter. ‘They are mostly hexagonal; yet there 
are a few four, five, and seven-sided prisms among the others. The 
top of the cliff is ike a flat pavement formed of large hexagonal blocks 
closely fitted together. The columns are imperfectly jointed, and 
break across with a flat surface, instead of a convexity. In certain 
parts of the cliff the top plane of the prisms is set obliquely, and the 
adjacent prisms over a large extent are alike and uniform in this obli- 
quity. These oblique terminations are in general confined to places 
consisting of oblique columns; the terminal planes are in all cases 
nearly horizontal. 
