which depend on surface conditions, and which may therefore be conveniently 

 referred to as the surface agencies. The extent of alteration produced by these 

 agencies varies from nothing to the complete disintegration of the rocks, and 

 the breaking-up of their constituent minerals into chemical compounds which 

 are more stable under the existing conditions. Under the influence of these 

 agencies the materials of certain rocks are removed, either in the soluble or 

 insoluble form, and re-deposited elsewhere. The deposition of the insoluble 

 material as sediment by the action of gravitation gives rise to the formation 

 of gravel, sand and mud. In order that the soluble material may again take 

 the solid form it is necessary that chemical or organic action should supervene ; 

 or that the solvent medium should evaporate. 



The rocks now forming under the influence of surface agencies are 

 characterised by an arrangement in layers, usually of small thickness but of 

 considerable horizontal extent. They are the result of the aggregation of 

 mineral particles under the influence of gravitation, and the planes which 

 separate the different layers stand related to the direction in which this force 

 acts and are, in the case of the finer deposits, approximately at right angles to 

 this direction. 



The changes which result from the operation of the above agencies are 

 not, however, limited to the actual surface. Meteoric water percolates to 

 great depths and produces directly and indirectly, by means of the substances 

 which it holds in solution, the most profound chemical and physical changes. 

 Oxidation, reduction, solution, hydration, decomposition, and re-composition 

 are constantly going on throughout the rock-masses, and are largely dependent 

 on surface conditions. Springs which issue from the interior are charged 

 with various substances in solution. These substances are the end-result, so 

 far as the water is concerned, of the complex chemical reactions which it has 

 been the means of producing in the rocks through which it has passed. They 

 represent, it must be remembered, a far greater amount of change than the 

 mere solution of so much solid material. 



In addition to the subterranean agents of change, which are more or less 

 dependent on surface conditions, there are others which are mainly due to the 

 high temperature of the interior. Masses of molten matter make their way to 

 the surface in connection with volcanic action ; hot water, acid and other gases 

 also rise to the surface from great depths under certain circumstances. The 

 operation of these agents on the rocks of the earth's crust cannot be directly 

 examined; but a considerable amount of information may be obtained with 

 reference to them by the application of known chemical and physical laws, and 

 by the observation of the effects produced in th'e past on rocks which have 

 been exposed at the surface by denudation. Contact metamorphism, produced 

 by the intrusion of masses of eruptive rock, is one of the most important 

 of these effects. It will be described in connection with the rocks themselves. 



Another cause which has certainly produced most important changes over 

 extensive areas is the lateral thrust of which there is such striking evidence 

 in many districts. It is now abundantly clear that the solid rocks, in certain 

 cases, yield as plastic bodies that is by the movement of the constituent 

 particles over each other under the influence of the powerful stresses which 



