418 



habit The zone of maximum dynamic metamorphism in the disturbed 

 area is the zone of contact between the crystalline rocks and the 

 sedimentaries. There is, moreover, an apparent transition from the 

 sedimentary rocks on the outskirts of the mountain-range to the more 

 or less foliated plutonic rocks occupying the central portions of the 

 range. This transition was regarded by the earlier geologists as 

 representing a transition in time a transition from a supposed prim- 

 ordial condition of things, when granite and gneiss were deposited as 

 sediments, to a condition when the globe was sufficiently cool to 

 admit of the deposition of ordinary sediment and of the existence of 

 organic life. This view is now rapidly disappearing in favour of the 

 idea that the sequence above referred to is a structural and not a 

 chronological sequence, except in so far as the oldest stratified rocks 

 necessarily lie, as a general rule, nearest the mountain axis. 



It must not, however, be supposed that wide-spread dynamic, or, 

 in other words, regional metamorphism is limited to what are now 

 great mountain ranges. It has operated in all regions of crystalline 

 schist and also in many districts largely composed of rocks belonging 

 to definitely recognizable geological periods in short, wherever rocks 

 have suffered great deformation by the earth-stresses. 



In dealing with the effects of dynamic metamorphism we must, 

 of course, remember that the plastic deformation of a mass of rock 

 can only take place under immense pressure, and also that the 

 mechanical energy expended upon the rock in the act of deformation 

 will be transformed into other kinds of energy in the mass of the 

 rock. It seems probable that the bulk of it will take the form of 

 heat as in MALLET'S^ experiments, and if so, a rise of temperature will 

 be the result. It does not by any means follow, however, that the rise 

 of temperature will be considerable. This will depend on the rate at 

 which the heat is generated by deformation as compared with that at 

 which it is dissipated by conduction. We are, therefore, at present 

 totally unable to estimate the elevation of temperature which accom- 

 panies the deformation of rocks in any given case. It probably varies 

 in different cases, and may sometimes be sufficient to melt (dissolve) 

 the rocks in the presence of water, seeing that water tends to lower 

 the fusing point of silicates. It seems tolerably clear from the occur- 

 rence of veins and segregations of quartz, quartz and albite, quartz and 

 orthoclase (pegmatite), and other mineral aggregates in areas of regional 

 metamorphism, that a migration and concentration of certain substances 

 is rendered possible under the conditions which produce this kind of 

 metamorphism. 



For our present purpose it will be convenient to consider dynamic 

 metamorphism under the following heads. 



(1) As it affects the structure of igneous rocks. 



(2) As it affects the composition of igneous rocks. 



(3) As it affects the relations of rock-masses. 



(1) Phil. Trans., Vol. 163 (1873) p. 147. 



