392 
PROFESSOR F. D. ADAMS AND DR. J. T. NICOLSON 
in the little shear zones of the artificially deformed marble. There are, however, a 
few rather coarser-grained streaks in the section, and these are composed of calcite 
grains which show marked twinning, and which are being broken down by granula¬ 
tion into minute grains like those composing the mass of the rock. These latter are 
seen under a very high power to be distinctly flattened, while the pigment still 
remains as minute black dots scattered throughout the mass. The somewhat 
coarser-grained streaks evidently result from the rolling out of little veins of 
calcite formed in the rock during the earlier stages of its deformation, as shown by 
the fact that they cut obliquely across the foliation of the rock in many cases- 
They consequently are free from pigment, but have been greatly crushed by later 
movements, and now consist of small calcite fragments in a finely granulated ground- 
mass, presenting a typical cataclastic structure. These fragments have precisely the 
same “ fibrous” structure as that seen in the calcite of artificially deformed marbles. 
The fact that these later veins have not been recrystallised would seem to indicate 
that the finer grained groundmass of the rock is still intact in this respect, and 
that the flattening of the minute calcite grains has probably been produced by the 
pressure to which the rock has been subjected, as it is in the case of the Carrara 
marble in the experiments described in this paper. 
6. Limestone. Films, Switzerland .—A very fine-grained bluish Upper Jurassic 
limestone, showing structures similar to those described in No. 5. 
7. Limestone. Griesbach, Erzgebirge , Germany —A light grey granular limestone 
or marble, rather fine in grain, with an indistinct banded appearance caused by the 
alternation of lighter and darker streaks or bands. Under the microscope the rock 
shows what is to all appearances a well-marked cataclastic structure. There are 
larger grains of irregular elongated form, with their longer axes lying in the same 
direction, and between them smaller grains which look as if they had been torn from 
the larger ones. Almost every grain, large or small, is highly twinned, often showing- 
two sets of two lamellm crossing one another. The twinning is usually in very 
narrow polysynthetic bands, often so narrow that the grains have a fibrous appear¬ 
ance, exactly like that in the artificially deformed limestones. Strain shadows are 
also common, but usually the grains are so highly twinned that the strain seems to 
have been relieved in this way. The larger grains are often as much as seven times 
as long as they are wide and are ragged in outline. The whole appearance of the 
rock indicates movement under great compression. The structures are exactly those 
seen in the deformed Carrara marble. The cataclastic structure, however, as in 
Nos. 10 and 11, has a more coarse-grained development than that produced arti¬ 
ficially. The original rock was composed of larger individuals, and the granulated 
material is not so finely triturated. The other structure, which consists of the 
deformation and flattening of the component individuals of the rock by twinning and 
movement on their gliding planes, is exactly like that seen in the Carrara marble when 
deformed at 300° C. or 400° C. In thin sections the finer-grained portions of this 
