Postelsia 
OD, 
Oo 
to 
of rock material and fault lines have developed 
(Plate XX VII). - These intruding masses and 
the displacements they have -effected in the 
original rock mean much as_ rock history is 
read, for they speak of uplift and downthrow; 
of crushing and tension; of the flow of molten 
rock material, and slow accretion from perco- 
lating waters. 
The tiltine of the rocks from therm omemeal 
horizontal position involves stress of great force. 
Blocks sometimes hundreds of feet in thickness 
are shoved over each other until they he shifted 
many feet from their original position. Evi- 
dence of such displacement is seen at many 
places in broken structures like dikes, veins and 
stratification bands. These all have their story 
to’ tell: 
The dikes are masses of eruptive gnocks 
(Plate XXVIII). Long and more or less contin- 
uous fissures in the shales and slates reaching in- 
determinable depths were filled by molten lava 
pushed by some displacing agent from subterra- 
nean reservoirs. They indicate a tensional force, 
first producing the fissures and a crushing suffi- 
