164 



formation of the present fissures, wliicli subsequently were filled from 

 that source which supplied so much mineral matter to other neighbor- 

 ing rocks in the form of impregnation. It is extremely difficult to de- 

 cide questions of this kind, involving so many different factors, after 

 having made any but the most complete investigations into the subject. 

 I therefore only offer this explanation as a suggestion, without any fur- 

 ther elaboration. 



