DEPTH OF ZONE OF FLOW IN THE EARTH’S CRUST 107 
ends of the transverse hole, was then removed, and the question 
as to whether there had been a constriction of the transverse hole 
was determined by attempting to pass the wire through it. 
In those experiments in which the element of heat was intro- 
duced, the tube with the enclosed column was placed before com- 
pression in a suitably constructed apparatus in which it could be 
maintained at a constant temperature, during the experiment, the 
temperature being measured by a properly calibrated thermo- 
electric couple. 
For the purpose of experimental investigation, it was important 
to select rocks which not only represent the chief types composing 
the earth’s crust but which also possess physical characteristics 
which render them suitable for experimental purposes, that is to 
say, rocks which are massive, rather fine in grain, uniform in 
character, and free from cracks and other flaws. 
Two rocks which fulfil these conditions excellently are:—(z) 
the Lithographic Limestone from Solenhofen, Bavaria; (2) the Red 
Granite from Westerly, Rhode Island. 
LITHOGRAPHIC LIMESTONE, SOLENHOFEN, BAVARIA 
This may be taken as representing the softer sedimentary type 
of rocks which are found more abundantly at or near the surface 
in the upper portions of the earth’s crust. The Solenhofen lime- 
stone is very fine in grain, very massive in character, and of a buff 
color. It breaks with a splintery or choncoidal fracture, and 
Heim speaks of it as one of the most brittle of rocks. A chemical 
analysis showed that the variety employed in these experiments 
consisted of carbonate of lime holding 3.52 per cent of impurities. 
It has a specific gravity of 2.603 when dry and is slightly porous, 
as shown by the fact that when allowed to remain under water in 
vacuo for 24 hours it absorbs 1.63 per cent of water. 
In order to ascertain the crushing strength of the rock as deter- 
mined in the manner usually employed in the case of building 
stones, etc., a series of six 2-inch cubes were very carefully sawn 
out of a block of the limestone, and having been accurately ground, 
were tested in compression, observing all the precautions required 
to attain the most accurate results. These determinations were 
