ON AN EXPERIMENTAL INVESTIGATION INTO THE FLOW OF MARBLE. 369 
carrying out his experiments on the perforation of rocks by means of explosives. 
Little cylinders of marble, granite, and other rocks, having a diameter of about 
2 centims., were cut in two vertically, the two halves tightly bound together again 
by means of wire, and inserted firmly in a tube connected with a chamber in which 
small charges of dynamite were exploded. The escaping gas completely perforated 
the rock along the line of contact of the two half cylinders. Under the influence of 
the explosive force the marble cylinders apparently became somewhat plastic, the 
wire binding them often leaving slight depressions on the surface when removed. 
The cylinder was also found to be somewhat shorter and stouter, the marble had lost 
its translucency and had become ojmque. Under the microscope, he says, it was seen 
to have been crushed to powder, and then squeezed together again into a solid mass. 
The cylinders also had a concentric structure induced in them around the central 
perforation. 
The several experimental investigations hitherto made on the flow of rocks (and it 
is believed that the summary given above outlines all the experimental work in which 
the rocks themselves were employed, the results of which have appeared up to the 
present time), while very interesting and instructive, are inconclusive and in certain 
cases apparently mutually contradictory. Neither has account been taken in any of 
these investigations of the rapidity with which the pressure was applied, of the 
temperature of the rock during compression, nor, except in a very few cases, of the 
duration of the pressure. Nor have we in any case an accurate description of the 
character of the rock before and after the experiment, or of the strength of the 
deformed rock, so that the actual nature of the effect produced by the pressure can 
be determined. 
II. Conditions to be reproduced in Experimental Work. 
It is generally agreed that three chief factors contribute to bringing about the 
conditions to which rocks are subjected in the deeper parts of the earth’s crust, where 
folding with concomitant flowing is most marked. These are :— 
1. Great pressure. 
2. High temperature. 
3. Percolating waters. 
With regard to the first factor it must be noted, that mere cubic compression will not 
produce movements of the nature of flowing, although it may produce molecular 
rearrangement in the rock. A differential pressure is necessary to give movement to 
the mass. Heim* has stated the conditions of movement, so far as pressure affects 
them, as follows :— 
“ Plastische Umformung geschieht also nur, wenn allseitig ein Druck wirkt, dor 
* ‘ Untersuelmng iiber den Mechanismus der Gebirgsbildung, 5 Band II., s. 91. 
VOL. CXCV.—A. 3 B 
