THE PROBLEMS OF GEOLOGY 541 



are within the limits of the force of cohesion. This naturally led 

 to the suggestion of a deep-seated zone of rock-flowage, in opposi- 

 tion to a zone near the surface, that of fracture. At the time this 

 conclusion was reached, no experiments had been made actually 

 showing the deformation of rocks under the conditions of the deep- 

 seated zone, but since that time Adams and Nicolson have deformed 

 rocks by flowage in the laboratory. 1 Thus observation of the geo- 

 logist, inference from the observation, and experimental work have 

 led to advance in the science of physics. 



For the present purpose the important thing is to observe that a 

 realization of the very diverse results which follow from deformation 

 under different physical conditions led to a satisfactory classification 

 of two great sets of phenomena which had been noted, but without 

 any reason being assigned why one occurs at one place and the 

 second at another place. Thus in the text-books of geology, joints, 

 faults, and folds were described. But there was no attempt to 

 explain why fracture occurred here, folds there, and in a third place 

 both. After it was realized that the great earth-movement makes 

 joints, faults, and other fractures at and near the surface, and 

 at depth, below these structures, other structures which have been 

 called folds, it was possible to reduce the gross deformation of rocks 

 to some systematic order under the principles of physics. There of 

 course remains the working-out of the precise physical conditions 

 which result in the various diverse phenomena. For instance, what 

 are the exact conditions of stress which result in the many complex 

 systems of joints? While progress has been made upon this and 

 other problems of gross deformation, a vast amount of work remains 

 to be done before the subject will be even approximately reduced to 

 order in the terms of energy, agent, and process. 



It has already been intimated that the subject of rock alteration 

 was in an even more unsatisfactory state than that of gross deforma- 

 tion. The particular alteration of this or that rock was given without 

 any adequate consideration of the geological, physical, or chemical 

 conditions under which the change took place. Thus there were 

 many thousands of descriptions of rock alterations, but no under- 

 standing of the reasons why the particular alteration for a given 

 rock found at a given place occurred. To make the matter worse, 

 almost every description of rock alteration was accompanied by 

 vague guesses as to the causes of the changes, the majority of which 

 were little short of grotesque. 



After it was appreciated that the gross deformation of rocks is 

 very different in an upper and a deeper zone, the question naturally 



1 F. D. Adams and J. T. Nicolson, An Experimental Investigation into the Flow 

 of Marble, Philosophical Transactions of the Royal Society of London, Series A, vol. 

 cxcv (1901), pp. 363-401. 



