PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 93 



Gabbro.] 



feldspar, but the " secondary " increment has a laminated structure and an irregular 

 outline, both due to the form of the original pyroxene. 



These secondary minerals seem to have been derived entirely from the change 

 in the diallage, for the feldspars are quite free from such change. There is some- 

 times a patch of chlorite surrounded by a feldspar grain, but in all such cases, so far 

 as observed, there are a few fine grains of secondary magnetite embraced in the 

 chlorite, showing that the place of the chlorite was originally occupied by a grain of 

 diallage; or such chlorite grains can be referred to the effect of a neighboring disin- 

 tegrated diallage. 



From the foregoing, however, it is not established that the great masses of iron 

 ore of the gabbro are of secondary date, though it appears probable. If such a gen- 

 eration be established by such evidence it is necessary to understand the term 

 secondary to apply to a date just subsequent to consolidation prior to cooling, when 

 chemical changes were liable to rapid production. The changes that have resulted 

 since cooling, so far as seen in this rock now at the surface, are nil; but they may 

 have been great prior to the removal of the weathered surface by the glacial period. 



One section examined. 



Age. The Cabotian eruptives of the Taconic. N. H. w. 



Nos. ID AND IB. GABBRO. (Globular masses, tueatliering from No. 1.) 



Duluth. At a point half way between Newson's quarry and the depot. Also seen on Michigan street, 

 where it has been cut by grading and forms a bluff about fifteen feet high. 

 Compare Nos. 4A and 1799. 

 Kef. Annual Report, ix, page 11; Bulletin ii, pages 73, 74; Annual Report, x, page 41. 



Meg. An extensive disintegration characterizes a belt in the gabbro seen at this 

 place and again on Michigan street. It is finer toward the east. At the point where 

 this specimen was obtained the masses were over a foot in diameter, with many 

 smaller. Evidently these larger masses, being fresh and like therock of No. 1, are of the 

 nature of "boulders of disintegration," as defined by T. Sterry Hunt. But in other 

 places the rock is in the nature of a gravel of disintegration, the small gabbro pieces 

 being about the size of apples and walnuts, though surrounded by much decayed 

 rock. The face of the cut on Michigan street presents much the appearance of a 

 conglomerate (plate P, figure 1, vol. iv). 



Mic. A thin section from No. IE presents the same characters as No. 1, but the 

 minerals are much decayed. 



One section examined. 



Age. Cabotian. N. H. w. 



