184 F. D. ADAMS EXPERIMENT IN GEOLOGY 



foliation being at right angles to the direction of the force producing the 

 movement. 



In all these cases, therefore, schistosity is produced by pressure or by 

 pressure combined with heat, but in the absence of moisture. 



The question of the part played by solution under pressure in the de- 

 velopment of the crystalline schists is one of great importance. Speaking 

 generally, when a crystal is strained its solubility on the strained face is 

 increased. Consequently a strained crystal in contact with a saturated 

 solution of any solvent dissolves on the strained faces and is redeposited 

 where there is no strain. It would thus seem that in the presence of 

 moisture great differential pressure, even at low temperatures, might if 

 long continued effect a gradual recrystallization of a massive rock with 

 the development of a foliated or schistose structure in a direction at right 

 angles to the direction of maximum pressure. Becke, Grubenmann, and 

 others have seen in this conjectural process the chief factor in the devel- 

 opment of great series of crystalline schists which are met with in the 

 Alps and elsewhere and whose structure they designate as "Krystalliza- 

 tionsschieferung." 



This process of solution which seems to have taken place, in some cases 

 at least, and to have resulted in the development of a schistose structure 

 in the rock, has not as yet been submitted to investigation by experiment. 

 The phenomenon attributed to it may have been produced by recrystalli- 

 zation induced by pressure and heat. 



Enough has been said to show that it is impossible in the present state 

 of our knowledge to determine in every instance the relative importance 

 of the role played by pressure, heat, and solution in the development of 

 a body of crystalline schists. Some progress has been made in this direc- 

 tion, but there is still a very wide and fruitful field open for experimental 

 work. It would be a much appreciated boon if by such investigation it 

 were possible to rescue our successors from that state of despair described 

 by Sharpe "as the first impression of an observer entering a district of 

 gneiss or schists in search of order in their arrangement." 



I have dwelt at some length on what is really a single great field of 

 experimental effort — that, namely, which has as its goal a correct under- 

 standing of the manifold phases of the action of pressure, heat, and solu- 

 tion as displayed in the mechanism of mountain-making and in the de- 

 velopment of the crystalline schists, which is another and accompanying 

 manifestation of the same agencies. 



There are, however, many other lines along which experiment in geol- 

 ogy has made notable conquests and in which brilliant results have been 

 achieved. 



