398 
PROFESSOR F. D. ADAMS AND DR. J. T. NICOLSON 
often present, but there has been no distinct alteration in the shape of the grains by 
pressure. The individual grains are, in some cases, very irregular in shape and often 
come together along more or less crenulated lines, and in No. 24 the peculiar inter- 
growth of separate calcite individuals described by Vogt* was observed. The 
structure of the limestones and marbles of this class is in fact quite different from 
those included in the first list, although they might readily give rise to such rocks as 
these, were they subjected to dynamic action under the required conditions. 
It will thus have been seen that the deformed limestones and marbles met with in 
nature, present in many cases at least precisely the structures developed in marble by 
artificial deformation. Among these are to be especially noted, in the first place, 
cataclastic structure ; and, in the second place, the twisting, elongation, and flatten¬ 
ing of the component calcite individuals either with or without the concomitant 
development of twinning and strain shadows, these latter phenomena being almost 
invariably seen in the larger individuals but less frequently observed in the very 
small grains, apparently on account of the very smallness of their surface. When a 
large, highly twinned and strained calcite individual is observed breaking down into 
a mass of smaller grains, it can be distinctly seen that each individual grain resulting 
from this granulation is so small that it is, in the great majority of cases, derived from 
a single twin lamella, and its surface is so limited that the strain shadow upon it 
would be scarcely noticeable. 
While, therefore, recrystallisation undoubtedly plays an important, and in many 
cases probably a chief, part in the great movements which are observed to have taken 
place in the limestones of contorted districts, this process is by no means the only one 
by which such movements are brought about. Many limestones under pressure in 
the earth’s crust flow precisely as metals do by deformation of the compressed grains 
and without the intervention of water or any other solvent. 
VI. Summary of Results. 
1. By submitting limestone or marble to differential pressures exceeding the elastic 
limit of the rock and under the conditions described in this paper, permanent 
deformation can be produced. 
2. This deformation, when carried out at ordinary temperatures, is due in part to a 
cataclastic structure and in part to twinning and gliding movements in the 
individual crystals composing the rock. 
3. Both of these structures are seen in contorted limestones and marbles in nature. 
4. When the deformation is carried out at 300° C., or, better, at 400° C., the cata¬ 
clastic structure is not developed, and the whole movement is due to changes 
in the shape of the component calcite crystals, by twinning and gliding. 
5. This latter movement is identical with that produced in metals by squeezing or 
* Loc. cit., p. 13. 
