J. Q. Goodchild — Augen-sfructnre and Einiptive Rocks, etc. 23 



whi'cli the pressure would be at a minimum for that depth from 

 the surface. Rocks affected in that manner must, almost of necessity, 

 undergo more or less shearing, and as it is well known that heat 

 travels through sheared rocks more readily along their planes of 

 schistosity than in directions transverse to them, higher temperature 

 prevails thei'e from that cause alone. But schistose rocks, subjected 

 to lateral pressure, must give way more readily along their own 

 planes of maximum structural weakness than along any others. 

 Consequently, even a slight separation of the rock along these 

 planes gives rise to a relief of pressure, which, by degrees, tends to 

 bring about a gradual fusion at the points where the relief of 

 pressure has been greatest. The fusing points attained in such a 

 case would bear a definite relation to the ratio between the initial 

 and final pressure while the temperature remained constant. 



If, now, the earth movements cease, or the temperature falls, the 

 fused portions of the rock gradually crystallise out, and eventually 

 appear as augen set in a sheared matrix composed of the very same 

 minerals which have not had any opportunity of returning to the 

 crystalline condition. This local fusion, through gradual relief of 

 pressure acting upon rocks that are in a potentially molten condition, 

 forms the starting point of some phenomena of considerable interest 

 in the history of rock structures. The two examples so far con- 

 sidered have been those of rocks consisting mainly of amphibolite. 

 In the one case, these have been sheared into phacoids by mechanical 

 action without any subsequent chemical rearrangement of the 

 minerals affected. In the other case, molecular rearrangement has 

 been superinduced upon the results of mechanical deformation, and 

 amphibolite augen have been formed. But in many cases the rocks 

 acted upon have been of much less simple composition, and have 

 included the constituents of more than one or two minerals. la 

 rocks of such nature the fusing point of any one mineral of the 

 compound may differ considerably from that of the others. Assum- 

 ing that three such minerals are present, which for the present 

 purpose may be denoted respectively by A, B, and C, the fusing 

 point of one of them — A, under given conditions, may lie below 

 that of B ; while B in its turn fuses, under the same conditions, at a 

 lower temperature than C. If now the entire compound be subjected 

 to a temperature which is sufficent under a low pressure to fuse C, 

 but which temperature is, at the same time, counteracted by heavy 

 pressure, so that not only C, but B and A, remain unfused, the 

 minerals will all remain practically unaffected. But if the tempera- 

 ture rises sufficiently to fuse A under the same pressure, or if, what 

 amounts to the same thing, the pressure is slowly and gently eased 

 off, then A may pass into the molten state, leaving B and as they 

 were. Similarly with a further relief of pressure, the temperature 

 remaining as before, B also may pass into a condition which permits 

 of its re-arrangement within the rock, C remaining still unaltered. 

 A gradual fall of temperature now permits C to impress its form 

 upon B as crystallization follows ; and in its turn. A, being the last 

 to consolidate, is moulded ophitically around both of its associates. 



