184 Lamplugh : On Movements in Rocks. 
In fact, whenever we study the condition ,of rocks in the 
earth’s crust we learn that their rigidity is only relative, and 
that even the hardest have behaved like plastic bodies when sub- 
jected to intense pressure. It is believed that at great depths, 
under the combination of pressure and heat, all rocks become 
plastic, so that there is a ‘zone of flowage,’ as it is called, 
always beneath us. Certain it is that when strata which have 
once been very deeply buried are laid bare at the surface, they 
generally exhibit what is known as ‘ schistosity,’ all the par- 
ticles having been dragged or squeezed into new positions. 
Sometimes among these schists there are what have originally 
been bands of rounded pebbles of very hard material such 
as quartz, and not infrequently all the little pebbles have been 
drawn out into pencil-like rods, or have been flattened into 
thin discs. 
The force required to make the rocks so lively as this 1s, 
of course, enormous. We may get some vague conception of 
it by calculation, and that is all. But even within the range 
of pressures at our command by mechanical means, it has been 
quite possible to illustrate on a small scale how some rocks 
may be deformed. Limestone has long been known to yield 
with comparative readiness under pressure, and this is strik- 
ingly demonstrated in some experiments recently made by 
Prof. F. C. Adams, of Montreal. To mention only one of these 
interesting experiments, Prof. Adams found that a short 
column of limestone, 1°56 inch long, when submitted to a 
pressure of about 140 tons to the square inch in a hydraulic 
press, was converted into a disc only ‘682 inch thick, while 
the limestone remained nearly as hard as before. 
I have mentioned that time is an important factor in all 
the movements within the earth; and we have reason to 
believe that most of the great contortions have been produced 
in the strata very slowly. But there are other effects which 
have probably been in most cases produced much more quickly. 
Among these are the well-known ‘faults,’ or dislocations 
of the strata along definite planes of fracture. The strain has 
been too sudden and too great for the rocks to sustain by bend- 
ing, so they have snapped and slid into new positions by relative 
displacement at the fault-planes. Generally the displacement 
has taken the form of a sliding down of the strata on one side 
of the fault relatively to the other, which implies that the 
tendency of the movement has been to pull apart or to stretch 
the rocks. Such are known as ‘ normal faults.’ Sometimes, 
however, the rocks on one side of the fracture have been pushed 
forward over the other, it may be for a distance of many miles ; 
and this, of course, implies that relief has had to be obtained 
from violent compression. These are called overthrust faults, 
and they are nearly always accompanied by severe contortion, 
crushing and alteration of the strata. 
Naturalist, 
