IP s t e I s ia 335 



cient to crowd the flowing rock into every crack 

 and cranny of the superjacent formations. When 

 crystallized this rock became so hardened that it 

 has resisted erosion more than the enclosing 

 country rock and it now stands like an ancient 

 ruined wall upon the dark shales and slates. In 

 age relations the dikes are younger than intruded 

 shales and older than all overlying rocks. To 

 him who has seen the base of Mt. Edinburgh 

 the origin of the dikes seems closely associated 

 with the out-thrust of that huge pile of granite. 

 During the compressing and tilting of the 

 shales as well as subsequent to the intrusion of 

 the dikes there were formed master joints and 

 minor joints, together with numberless cracks 

 anastomosing in every direction. These open- 

 ings afforded surfaces on which the dccom- 

 position processes, yielding large quantities of 

 quartz and less quantities of other mineral sub- 

 stances, deposited their minerals. This material, 

 thus again deposited, filled these cracks and re- 

 cemented the rock into a solid mass. Thus the 

 veins were formed which present so striking a 

 feature of many a rock surface. They are al- 



