GROWTH OF EXPERIMENTAL SCIENCE 181 



the deformation was distinctly facilitated by heat (350° C. to 450° C). 

 The structnrej however, was no longer developed by a movement on the 

 gliding planes of the constituent minerals, bnt was produced chiefly by a 

 granulation of the minerals and the alignment of the granules in the 

 plane of the schistosity thus developed. The structure is identical with 

 that seen in mylonite and many augen gneisses, so that experimental 

 proof is afforded that such rocks may be produced in nature by the action 

 of pressure alone, but that the development of the structure is more 

 readily effected if the rocks are hot. 



So far as these types of rock are concerned, then, schistosity or foliation 

 may be produced by pressure alone, without heat and in the absence of 

 moisture. 



To recur for a moment to the deformation of marble. 



It will be found that when most of the foliated or schistose limestones 

 which occur in nature are examined under the microscope, they show 

 little or no evidence of pressure phenomena, such as those above described. 

 The rock frequently presents the appearance of a mosaic of equidimen- 

 sional grains, such as might be expected to have been developed by an 

 original crystallization of the material. The schistosity in these cases is 

 produced by an alternation of thin laminas of larger and smaller indi- 

 viduals of calcite or by the parallel arrangement of other minerals exist- 

 ing as impurities in th'e rock. 



A schistosity of this character, which is to be observed not only in lime- 

 stones, but in very many other crystalline schists, has by most authors 

 been regarded as susceptible of explanation only as being due to recrys- 

 tallization of the rock, through the agency of moisture present in the 

 rock as the deformation was going forward. Some very interesting ex- 

 perimental work which has been carried out in recent years in the 

 Geophysical Laboratory in Washington has, hov^ever, a very important 

 bearing on this question. 



Reference has already been made to the experiments of Sir James Hall, 

 in which he obtained a white crystalline marble by heating chalk under 

 high pressure, experiments which were repeated later by Gustave Rose 

 and the results confirmed. Hall believed that this change was due to a 

 partial or complete fusion of the marble with subsequent crystallization 

 on cooling. 



It has recently been found, however, that if finely powdered or loose, 

 finely crystalline aggregates of a mineral which is not destroyed by heat 

 are submitted for a considerable time to a temperature which, while high, 

 is still considerably below the fusion point of the mineral, the pov^'^der 

 will become progressively coarser in grain. The mineral slowly volatil- 



