Geology Reference
In-Depth Information
(c)
Figure 1.8.
(c) Vertical air photograph showing orthogonal fracture systems in granitic rocks in central
Labrador (Department of Energy, Mines and Resources, Canada).
refers to the capacity of a medium to transmit fluids. It differs from porosity for voids may be uncon-
nected, or be too narrow to allow transmission of fluids because of surface tension. Fresh granites are
of low permeability but most weathered rocks allow ready passage of fluids.
Perviousness is also known as secondary or acquired permeability and refers to the capacity of
a rock to transmit fluids not through the rock mass, but by way of voids, fractures and fissures.
Perviousness varies not only with the number of fractures per unit volume but also with their con-
dition - open or hairline (tight) - and according to whether they are connected.
Quarrymen working granite have long recognised that the rock splits more readily in certain direc-
tions than in others (Dale, 1923) (
Fig. 1.7b).
In the vertical or near-vertical plane three directions are
recognised. Rift (Tarr, 1891) is the direction of easiest splitting, grain is fairly easy, and hardway the
most difficult. Lift is a horizontal plane of easy splitting. Interpretations of these directions vary but
many consider that rift consists of microfissures and crystal dislocations related to tectonic stress. In
addition to these planes of weakness, visible partings or fractures are also well-developed. Fractures
along which there is no detectable dislocation are called joints, whereas those along which there has
been relative movement are faults. It is frequently difficult to prove dislocation along fractures in gran-
ite (though it is possible to demonstrate small displacements by matching crystal boundaries on oppo-
site sides of partings), and many fractures termed joints are, it is suspected, really faults. The
importance of fractures as a means of water penetration and transmission is enhanced by the low poros-
ity and permeability of fresh granite.
Fractures or partings that run in parallel are called sets, whereas repeated patterns (which comprise
combinations of different sets) are known as systems. Fracture patterns in granite are frequently
