Vol. 58. | ANNIVERSARY ADDRESS OF THE PRESIDENT. xxvii 
twinning and gliding—that is without crushing; but that, when the 
deformation takes place at about 400° C., no cataclastic structures 
are produced, the whole of the movement being due to twinning 
and gliding. It would perhaps clear the air if, instead of speaking 
of dynamé-metamorphism when treating of the origin of certain 
erystalline schists, we were to employ the term thermodynamic 
metamorphism. At any rate, the reference to rocks which have 
been powerfully affected by dynamic action and yet not crystallized, 
as well as to others which are holocrystalline and devoid of cataclastic 
structure, would be seen at once to have no bearing whatever on the 
theory which Lossen, Lehmann, and others have advanced. The 
fundamental conception underlying this theory is that solid rocks, 
such as gneiss, granite, quartzite, and limestone, may undergo deform- 
ation—may be folded, kneaded, and stretched like wax or clay under 
the influence of the earth-stresses, without ever passing into the 
magmatic condition. The observations of Baltzer, Heim, Reusch, 
Lehmann, Lapworth, and many others have placed this fact beyond 
all possibility of doubt. Prof. Lehmann’s work must for ever remain 
a classic, so far as this subject is concerned, on account of the 
wonderful atlas of photographs with which it is illustrated. Words 
are after all but a poor substitute for the facts of Nature, and 
sketches not unfrequently illustrate the views of the author or the 
artist, rather than the objects which they are intended to represent ; 
but a properly taken photograph often shows almost as much as the 
object itself, and gives the reader an opportunity of judging of 
the correctness of the author’s conclusions. The photographic 
illustrations accompanying Lehmann’s work enable the facts to 
speak for themselves. | 
The deformation of a rock-mass can only be produced by a relative 
movement of some, or all, of its parts. Two extreme cases may arise: 
the strain may be distributed throughout a considerable mass of 
rock, in which case the microscopic character of the rock will be 
altered; or it may be localized along special planes, in which case 
the parts enclosed between these different planes will retain their 
original characters. At first sight, it appears as if there were a 
radical difference between these two types of deformation; but, as a 
matter of fact, we find in Nature a most perfect gradation between 
them. Deformation of the first type is usually referred to as 
plastic defor mation, although the use of this expression is not 
intended to imply that the individual minerals have changed their 
form without changing their character or losing their individuality ; 
