16 Alfred Harker—Physics of Metamorphism. 
indeed evident that what is essential to contact-metamorphism is not 
the proximity of an eruptive mass, but simply an elevation of 
temperature ; and, so far as chemical and mineralogical changes are 
concerned, it is immaterial whether the heat be conducted from some 
neighbouring source or generated in situ by mechanical or other 
means. It appears, again, from the writings of many geologists who 
invoke the operation of mechanical force in explanation of meta- 
morphism, that they require nothing of that agent beyond the 
generation of heat by the crushing of the rocks operated upon. A 
theory based on these lines fails to show any cause for difference 
between the results of dynamical and of contact-metamorphism. 
That differences of kind exist, is, however, generally admitted. 
Certain minerals, such as andalusite, tourmaline, garnet, and idocrase, 
are known to be especially associated with the alteration of stratified 
rocks at high temperatures. On the other hand, there are changes, 
such as the amphibolisation of pyroxene, the saussuritisation of 
plagioclase, the conversion of potash-felspar into white mica and 
quartz, and the production of sphene at the expense of titaniferous 
iron ores, which are characteristic of altered rocks showing clear 
evidence of mechanical stress. These two phases of metamorphism 
by no means exhaust the possibilities, but they apparently compel us 
at the outset to recognize as distinct from one another a thermo- and 
a dynamo-metamorphism, the pressure in the latter case operating 
not through the medium of heat generated, but immediately. In 
short, we must admit the direcé correlation between mechanical and 
chemical energy, to which experimental results unmistakably point. 
While allowing the potency of pressure as a factor in geological 
transformations, many writers seem reluctant to admit it to a rank 
co-ordinate with temperature, as one of the conditions governing all 
chemical processes. They quote the experiments of Mallet, but 
overlook those of Cailletet and Spring. But if the chemist can, for 
the most, neglect in practice the effects of variations of pressure on 
solid bodies, it is only because the range of pressure in his experi- 
ments is comparatively small: the laboratory of nature knows no 
such restrictions. In parts of the earth’s crust with which geological 
research is concerned, there must be enormous pressure, as well as 
extreme temperature; and these two, though often locally coinciding, 
must be physically independent. For rough purposes we may 
separate four sets of conditions, which may exist in various places, 
and which may be expected to govern the atomic forces in different 
manners, giving rise in their several provinces to different chemical 
combinations : 
(i). Low temperature and low pressure (hydro-metamorphism). 
(ii). High temperature and low pressure (thermo-metamorphism). 
(iii). Low temperature and high pressure (dynamo-metamorphism). 
(iv). High temperature and high pressure (p/atono-metamorphism). 
The first, in the widest sense, will embrace various chemical 
changes which go on chiefly near the surface of the earth. The 
second will be typically represented by ‘contact-metamorphism,’ 
but will also include cases in which the crushing of rocks under a 
