GROWTH OF EXPERIMENTAL SCIENCE 179 



a mass of pure wax differential pressure will develop a well defined schis- 

 tose structure through the flattening in one plane of the minute globules 

 of which that material is composed. But the question as to whether pres- 

 sure alone could bring about such movements in the solid rocks of the 

 earth's crust remained unanswered. 



David Forbes/^ as early as 1855, carried out a series of experiments 

 in which slabs of rock were embedded in the floor of a blast furnace, and 

 from these concluded that not only pressure but also heat, which effected 

 a partial recrystallization of the constituents of the rock, was needed to 

 produce foliation. 



A school of later observers, again, have held that the foliation of the 

 crystalline schists is a phenomenon due to a process of solution and re- 

 deposition of the constituent minerals of the rock under conditions of 

 pressure in the presence of moisture. 



While, therefore, all observers agree that pressure is an agent in the 

 production of schistosity or foliation, the relative part played in this 

 process by pressure, heat, and solution has remained a matter of indi- 

 vidual opinion. 



When mapping the very extended area of Precambrian rocks embraced 

 by the Haliburton and Bancroft region of the Laurentian peneplain in 

 southeastern Ontario, this ancient question as to the part played by these 

 three factors respectively in the development of the foliated or gneissic 

 structure, which is seen almost ever3^where in more or less pronounced 

 development over these thousands of square miles of glaciated exposures, 

 continually presented itself. 



It was impossible to solve the question by the closest and most atten- 

 tive observation in the field or by the most careful petrographic study of 

 thin-sections of the rocks themselves. It seemed, however, that some 

 light might be thrown on the problem by the aid of further experiment, 

 for in experimentation it might be possible to separate the three factors 

 of pressure, heat, and solution and investigate the action of each sepa- 

 rately. 



It was evident from the field study that the limestones of this ancient 

 complex were the most plastic element in the series and yielded most 

 readily to the forces which had produced movement with its concomitant 

 development of schistosity. It appeared, therefore, that this rock would 

 lend itself most readily to trial by experiment. 



Pressure alone was first employed. And in order to reproduce the con- 

 ditions of pressure found in the earth's crust, a differential pressure — 



" On the causes producing foliation in rocks. Quart. Jour. Geol. Soc, 1855, p. 184. 



