Characteristics and foundations - Landforms and Geology of Granite Terrains - page 12

(b)

(c)

Figure 1.6. (b) Porphyritic granite, South East district of South Australia. (c) Thin section of gneissic granite

with augen (“ eyes ”) of potash feldspar and plagioclase set in layered matrix of biotite and quartz;
scale in mm.

which there has been some dislocation. Such lineation is much more pronounced in gneisses (Fig.
1.6c), where there is, in addition, a tendency for minerals to form distinct layers, or folia, producing
distinct planes of splitting or foliation ( Fig. 1.7a). Planes of easy parting known as cleavage and
schistosity (and also strain in individual crystals, and notably in quartz) are caused by brittle-state
fracture. These features markedly influence the penetration of water into the rock mass and thus the