SILURIAN. 
165 
primitive hills. We are thus introduced 
into the domains of pure physical force, un¬ 
accompanied by any traces of life, save the 
evidences of the exercise of mighty power 
which the masses themselves everywhere 
display. 
The student must not, however, conclude 
from such enumerations that the old hard 
rocks are easily reduced into system and 
known at first sight. Though existing in 
substance so considerable that one groupe, 
the Silurian, in one place, is estimated to 
be a mile and a half thick, yet the detection 
of their true order and character is by no 
means easy. The striking mineral differ¬ 
ences which distinguish the members of the 
higher formations, the clay, chalk and 
oolite, do not obtain among the severe as¬ 
pects of the slate rocks. Hence when we 
find only one of the latter exhibited, and 
that, either totally, or in a large area devoid 
of organic remains, it is with difficulty that 
its geological position is determined; and 
thus the downs and defiles of Cornwall, 
Wales, and the Rhine-land, have been the 
scenes of protracted modern warfare be- 
