ABSTRACT
The structural mapping of igneous massifs received a great stimulus when Hans Cloos suggested ways in which flow structures, joints, and faults might be systematically related to the shape of intrusions and their mode of emplacement. 1 According to Cloos, intrusions may be considered firstly as fluids which move under pressure in chambers within the crust, which cham- bers may also be enlarged and moulded by the intrusive processes. During this stage flow structures are formed. In the second stage when the intrusion has cooled somewhat, a two-phase system is produced consisting of crystals forming a mesh-work with residual liquid between the grains. This is capable of sustaining shearing stress and may yield by faulting or by the opening of tension gashes. At this stage, during which forces are still operating to enlarge the intrusion and to force the walls apart, structures due to fracture are first formed, and they continue to form while the rock solidifies. Finally, after complete solidification, further fracturing due to crustal stresses and also to cooling stresses may be superimposed on the earlier, but in Cloos’ synthesis these post-solidification structures, which previously had been thought to be chiefly responsible for the jointing of granites, are of minor importance. These notions find their most fruitful application in the study of intrusions of plutonic type (plutons), but they also, afford a basis for the study of all igneous rocks and, with some modification, to zones of granitization or transfusion.
