332 IP s t e I s i a 



of rock material and fault lines have developed 

 (Plate XXVII). These intruding masses and 

 the displacements they have effected in the 

 orif^inal 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 tilting of the rocks from their original 

 horizontal position involves stress of great force. 

 Blocks sometimes hundreds of feet in thickness 

 are shoved over each other until they lie 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 rocks 

 (Plate XX\TII). 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- 



