MUSKUM 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 j)n)to<jim We accordingly find 



the sienitcs 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 bo 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 ue.}dtes 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 

 cliango 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 mctamorphism which has taken place 

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

 causes, whicl^ 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 

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

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

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

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

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

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

 the oldest paleozoic rocks. Previous to this. Dr. John Locke attemp'tcd 

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

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



attention to his bathetic description of the " Pictured Eocks " Cpn 189 ^ 

 191). ^^^* 



* Senate Docunionts, 2(1 Scss. SOih Cong., 1848-49, III., Doc. 2, p]). 159-1G3 ; 

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



t Senate Documents, Ist Sess. 30tk Cong., 1847-48, II. 180-189, Oct. 25, 1847. 



