848 REPORT—1885. 
formation of white mica and quartz by the destruction of orthoclase is 
probably another significant metamorphosis ; muscovite, like hornblende, 
has never been reproduced artificially. The mode of occurrence of 
hydrous micas, frequently associated with slickensides and other imme- 
diate evidences of stress, seems to point to the importance of mechanical 
forces in their genesis. Professor Bonney! considers that ‘ these filmy 
minerals appear to be very readily formed under pressure from damp 
argillaceous material in a state of fine division,’ and is of opinion that 
* perhaps it is hardly too much to say that the difference between a satiny 
slate or phyllite and an ordinary shale is due even more to the action of 
pressure than to mineral composition or geological age.’ The same 
authority, however, insists on an essential distinction between such rocks 
and true schists, which latter, he maintains, require for their production 
something more than mechanical forces. 
In weighing the relative importance of the results of mechanical stress 
on the one hand, and the more direct effects of the central heat of the 
earth on the other, it must not be overlooked that not only increased 
pressure, but also rise of temperature may result from the former agency. 
‘ The heat produced locally within the crust of the earth by transformation 
into heat of the mechanical work of compression, or of crushing of 
portions of that crust,’ as in Mr. Mallet’s* experiments, has been invoked 
by Professor Prestwich,* in a recent paper, as a factor of the first im- 
portance in the metamorphism of certain regions, such as the Appalachian 
Mountains and the Ardennes. This transformation of the mechanical 
work done by pressure into heat is, of course, quite distinct from the 
direct effects of pressure on physical and chemical forces discussed above. 
The latter necessitates no rise of temperature, but involves an immediate 
correlation between mechanical work and the energy of molecular and 
atomic forces. The heat in the former case, moreover, would arise partly 
from the hydrostatic pressure and consequent compression, partly from the 
shearing stress and associated deformation ; and in hard rocks the latter 
would doubtless be of the most importance. It seems reasonable to 
suppose, then, that the work done upon a rock by a lateral pressure to 
which it yields is expended in three ways—viz., in producing deformation 
of the rock mass both by plastic shearing and by fracture, in bringing 
about molecular and chemicai changes in its composition, and in gene- 
rating heat, which will again give rise to changes not always of the same 
kind as the former. The proportion of the available energy devoted to 
each of these effects must naturally depend upon the nature of the rocks 
operated upon. 
Considerations such as these may perhaps serve in some measure to 
lessen the difficulties that beset the study of cleavage and foliation by 
referring apparent anomalies to the different lithological characters of the 
rocks affected. Thus in Mr. Teall’s® dolerite dyke molecular rearrange- 
ment is seemingly an earlier effect of the forces concerned than foliation- 
structure, while, in Professor Bonney’s Torcross section, the reverse is 
the case. In an originally soft rock, a deformation or flowing, with its 
1 Quart. Journ. Geol. Soc., vol. x1. pp. 18, 25, 26 (1884). 
2 «On Volcanic Energy,’ Phil. Trans., vol. clxiii. p. 147 (1873). Cf. Daubrée, 
Etudes Synthét. de Géol. Expérim., p. 448 et seg. (1879), Paris. Bull. Soc. Géol. Fr., 
1878, p. 550. 
3 Roy. Soc., June 18,1885. Nature, July 2. * Sorby, doc. cit. 
5 Loc. cit., pp. 139, 142. 
