ON AN EXPERIMENTAL INVESTIGATION INTO THE FLOW OF MARBLE. 381 
halves of the tube without splitting in two as in all other cases ; but unfortunately it 
was traversed by a few slight cracks developed in the process which rendered it 
impossible to test its strength. The surface where it had been in contact with the 
hot collar was brown in colour and maintained its polish, except around the central 
zone of maximum bulging, where the lustre had disappeared. There had been no 
disassociation of the calcium carbonate, as fragments of the powdered marble tested 
with moist turmeric paper gave no trace of an alkaline reaction. When sliced and 
examined under the microscope, the rock showed no trace of cataclastic structure, 
hut the grains were seen to he distinctly flattened, giving to the rock a foliation which 
in some places was very pronounced. The calcite individuals showed the very narrow 
polysynthetic twinning producing the fibrous appearance before described. The twin 
lamellae are in some cases twisted, the twisting being accompanied by strain shadows, 
which phenomenon, however, in this rock is neither very common nor very striking. 
The individuals are seen in many cases to have been squeezed into very irregular 
shapes, and in some cases to have been forced into wedge-shaped forms (fig. 6), quite 
different from those of the regular mosaic of the original rock. The individual 
grains have to all appearance acted as plastic bodies. A very pronounced movement 
along gliding planes, coinciding in direction with the course of twin lamellae, is 
undoubted. 
Apart from the evidence of this presented by the form of the calcite indi¬ 
viduals, direct evidence can he seen in many cases in the step-like outline of the 
grains, as shown in fig. G, the steps coinciding in direction with the twin lamellae. In 
one instance, shown in fig. 7, a lamella was seen to have moved inward between 
two other very narrow lamellae on either side. This is of especial interest, as it is 
precisely this movement of individual lamellae of measurable width over one another 
