Alfred Harker—Physics of Metamorphism. 17 
moderate pressure gives rise to the generation of heat sufficient to 
produce a considerable rise of temperature. The third set of con- 
’ ditions will be best exemplified when pressure acts on a rock-mass 
not rigid enough to offer great resistance to crushing. A good 
illustration of its effects is seen in satiny slates or phyllades, such as 
those of Fumay, and perhaps of Llanberis and Michigan. These 
rocks, distinguished from mere clay-slates, must in some cases be 
classed as highly metamorphic. ‘They appear to consist largely of 
authigenic minerals, and their highly-developed cleavage-structure 
is in fact a micro-foliation. 
By applying to various districts of altered rocks this distinction of 
thermo- and dynamo-metamorphism, that is, by considering the 
proximate instead of the mediate causes of the chemical trans- 
formations, we may possibly find a clue to some apparent anomalies. 
One of these is the interpolation of thermo-metamorphic rocks in 
the midst of a region of dynamo-metamorphism. Such are the 
puzzling cornéites (composed of biotite and quartz), the garnetiferous 
and other special rocks of the Remagne and Bastogne district in the 
Belgian Ardenne. Their peculiarity seems readily to connect itself 
with the facts that they occur in the cores of anticlinal folds, where 
they must have been crushed upon themselves, and that the rocks 
thus affected, unlike their neighbours, are hard beds which would 
offer resistance to the deformation, and so give rise to the generation 
of heat. 
It is not to be expected, however, that the varied mineralogical 
phenomena of a great metamorphic region will in general divide 
themselves into those due to high temperature and those due to high 
pressure. We have still to reckon with the case of great heat 
produced by the crushing of large masses of hard rocks under a 
pressure itself sufficient to modify materially the chemical affinities. 
The results produced by the superposition of thermo- and dynamo- 
metamorphism must be very complex: but we have no reason to 
suppose that these are comparable in any special way with the effects 
of a very high temperature and an intense pressure operating simul- 
taneously. Indeed we must recognize in this latter a problem to 
which we can bring no direct experimental knowledge. Rocks 
which have been subjected to such conditions may presumably 
assimilate in some respects to those solidified from fusion under 
similar circumstances, although we should expect differences of 
textural and structural characters between the two. Failing a 
better name, we may accordingly use the term plutono-metamorphism 
to describe the profound changes in rocks implied in the joint 
influence of very elevated temperature and enormous pressure. 
Experiments in the artificial reproduction of minerals have led 
geologists to lay stress on the presence of water during the trans- 
formation of rocks. This question does not touch the fourfold 
division suggested above. ‘The temperature and the pressure must 
in general’ completely define the conditions of the chemical pro- 
* The exceptions being fusion and solidification, which cannot fairly be classed 
with metamorphism, 
DECADE Iil.—VOL. VI.—NO. I. 2 
