8 



deform the rock masses. In some cases the movement is accompanied only by 

 a mechanical re-arrangement of the constituent particles, in others by a crushing 

 of these particles, and in others by a molecular re -adjustment. In all cases new 

 structures and in many cases new minerals are developed; so that the original 

 characters of the rocks are more or less obliterated. The term " regional 

 metamorphism " will, in the present work, be applied to the action above 

 referred to. (1) 



There yet remain to be considered the rock-producing agencies which 

 depend upon internal heat. The only agencies of this kind which can be 

 directly observed are those connected with volcanic action. Masses of molten 

 material, or, more frequently of molten material charged with crystals, are 

 blown into the atmosphere by the explosive escape of contained gases and 

 vapours, or else well out from the crater or from a fissure in a comparatively 

 quiet manner as a lava stream. The final consolidation of the fragments! 

 material, after it has fallen on the surface, or of the lava current, gives rise to 

 a new rock, entirely different from the rocks already referred to as originating 

 from the destruction of older rocks by surface agencies. New rocks may also 

 arise in consequence of the consolidation of molten material beneath the surface 

 in large masses, veins, and dykes. 



We recognise then, two strongly contrasted groups of rock-forming 

 agencies now in operation : (1) those which depend 011 surface action and give 

 rise to sedimentary, chemical and organic deposits, and (2) those which 

 depend on internal action and give rise to fragmental volcanic rocks, 

 lavas, and crystalline masses of deep-seated origin. We recognise also that 

 the rocks already formed are constantly undergoing change in con- 

 sequence of changes in their environment. In this way rocks possessing 

 new characters may be produced from pre-existing rocks without the disinteg- 

 ration of the latter by surface agencies, or without their reduction to the molten 

 condition by intense heat. The agents of change may be conveniently 

 classified as follows : 



(1) Those which depend mainly on surface actions ; e.g. percolation of 



water charged with various substances in solution. 



(2) Those which depend mainly on internal heat ; including the action 



of molten material, hot aqueous solutions and acid gases. 



(3) Those which depend mainly on intense mechanical stresses operat- 



ing upon solid rocks in the earth's crust regional metamorphism. 



(1) The conversion of argillaceous deposits into clay-slate, quartzite into quartz schist, 

 ordinary limestone into crystalline marble (Baltzer, der Mechanische Contaet von 

 Gneiss und Kalk. Berne, 1880, pp. 51 60), and dolerite into hornblende-schist (Teall, Q.J G.S. 

 Vol. XLL, p. 133), may be quoted as illustrations of this action. 



The different kinds of metamorphism may be superposed, or the same kind of metamorphism 

 may affect the same area at different periods. Thus in the Bergen Peninsula ;sce Rensch, Die 

 fossilien fiihrenden krystallinischen Schiefer von Bergen in Norwegen, 1883), there are crystal- 

 line schists, probably of pre-cambrian age, lower paloeozoic sediments and eruptive musses of 

 granite, gabbro, &c. All these rocks have been simultaneously affected by post-silurian regional 

 metamorphism; the direction of the most intense pressure being N W. and S.E. The effect of 

 this pressure has been to produce profound structural and mineralogical changes in the rocks of 

 the district, and to impress on the bedding, cleavage and foliation, a dominant N.E. and S.W. 

 strike; so that a continuous stratigraphical sequence is simulated in what is, in reality, a very 

 complex mass. 



