MUSEUM OF COMPARATIVE ZOOLOGY. 5 



becomes the predominant mineral, constituting what may be denomi- 

 nated a quartzite In a few instances talc was found to take the 



place of hornblende, constituting a iirotogine We accordingly find 



the sienites assuming a trappose character, and often undergoing so in- 

 sensibly the change from a granitic to trappean rock, that it is impossible 

 to distinguish where one begins, and the other ends. In the operation 

 'of these changes, the excess of silica may be called in to account for 

 the metamorphic rocks of the country, and particularly for the abun- 

 dance of pure quartz in rocks and veins Not only have 



changes accompanied the contact of trap with other rocks, such as 

 have usually been referred to the heat of the injected mass when 

 in a state of fusion, but equally marked changes have accompanied 

 the conjunction of the sienites with the sandrock under circumstances 

 where the same causes cannot be called in ; for the latter gives evidence 

 of having been deposited subsequently to the formation and uplift of 

 the former, and the sienite was as often observed to have partaken of the 

 change as the sedimentary rock. Without going further into detail of 

 facts of merely scientific nature, it may be sufficient to say that it seems 

 more reasonable to attribute the metamorphism which has taken place 

 in both rocks rather to galvanic and chemical action than to igneous 

 causes, which are so generally called in to account for all these phenom- 

 ena. The rocks designated upon the maps as metamorphic, occupy, 

 as it were, beds amid the surrounding primary rocks ; and while we 

 would avoid any theoretic conclusions as to their origin, it may be stated 

 that, throughout the whole primary region, the limits of each rock, 

 except in the case of dikes, are seldom distinctly defined, but one passes 

 into the other by gradual transition ; so that often rocks of distinct 

 name and character can be considered only as members of the same foi'- 

 mation, the constituents of which have become differently aggregated." 

 Mr. J. W. Foster, in his report to Dr. Charles T. Jackson, dated Sep- 

 tember 28, 1848,* describes part of the iron region. The rocks are con- 

 sidered to be older than the sandstone, which is regarded as belonging to 

 the oldest palseozoic rocks. Previous to this, Dr. John Locke attempted 

 to describe the district in part ; but his account appears to be of no 

 value, except from an historical point of view.f We would, however, call 

 attention to his bathetic description of the "Pictured Rocks " (pp. 189 - 

 191). 



* Senate Documents, 2cl Sess. 30tli Cong., 1848-49, III., Doc. 2, pp. 159-163 ; 

 Executive Documents, III., Doc. 12, pp. 159-163. 



t Senate Documents, 1st Sess. 30th Cong., 1847-48, 11. 186-189, Oct. 25, 1847. 



