48 THE CRYSTAL FALLS IROX-BEARING DISTRICT. 



presumed to be secondary, but this can not l)e proved for them. The 

 schistose character of tlie rocks is evidence of dynamic action. The pres- 

 ence of undulatory extinction was noticed in the quartz of some s^^ecimens, 

 but .its absence is the rule. However, from the absence of great pressure 

 phenomena, and the remarkably fresh condition of the minerals composing 

 the basic rocks, which contrasts strongly with the generally altered condition 

 of the minerals of the more refractory acid rocks including them, it would 

 appear that complete recrystallization has occurred.-^ 



The schistose structure can uudoubtedl}^ be referred to the dynamic 

 action which resulted in the upturning of the sedimentaries and caused the 

 development of schistosity in certain portions of the border of the granite. 

 This dynamic action was in all probability also the chief force in the pro- 

 duction of the secondary minerals. 



The schistosity of the dikes does not agree in direction with the gen- 

 eral strike of the schistosity throughout the entire district, but is always 

 nearly parallel to the long extension of the dikes. These dikes represent 

 belts of weakness, and it is therefore natural that the movements should 

 occur along these belts rather than across them. 



This schistosity of the dikes also furnishes a slight clue as to their age. 

 Younger than the granites they cut, they must have occupied their present 

 position at the time the dynamic revolution took place which resulted in 

 the development of schistosity in the granite, as well as in the sedimentaries. 

 It is impossible to bring 'the date of their intrusion within narrow limits. 

 It seems very probable, however, that they were formed at the time of the 

 extrusion of the basic Hemlock volcanics, though it is impossible to prove 

 their connection with them. 



(2) MASSIVE DIKES. 



The only dike rock which retains to some extent its original texture is 

 a much-altered medium-grained dolerite (diabase). The alterations it has 

 undergone are those usual for such basic types of rock, and this one exhibits 

 nothing peculiar or of special interest. An ophitic texture, while still recog- 

 nizable, is more or less obscured b}^ the uralite which has developed out of 

 the pyroxene. The remnants of the original plagioclase feldspar present 

 show exceedingly slight pressure effects. The alteration processes would 



'Principlea of North American pie-Caiiibrian Geology, cit.,pi). 706-707. 



