% THE GEOLOGY OF MINNESOTA. 



[Diabase. Gabbro. 



porphyritic aspect. Areas of a soft, dark-green mineral, which seems to be chlorite, 

 are common, as are also small particles of epidote. Magnetite is also present. 



Mir. This rock is much decayed. The chief minerals present are feldspar, 

 chlorite, epidote and magnetite. The feldspar is largely altered to a mass of minute, 

 grayish, sericite-like fibres, but in many places enough of the original mineral remains 

 to show that it possessed frequent twinning lamellae. The fibrous alteration product 

 is sometimes arranged in groups of fibres radiating from a point or from a line. The 

 feldspar crystals are partially idiomorphic, and in the spaces between them are some 

 angular areas now filled with chlorite and fine grains of magnetite. The rock thus 

 appears to have been a diabase with the augite now represented only by the chlorite 

 areas. These are, however, in comparatively small amount, as the feldspar makes 

 up the great part of the rock. Epidote has developed to some extent in the feldspar; 

 it occurs in granular aggregates and irregular areas and is colorless to straw colored, 

 the more colored parts being somewhat pleochroic. Magnetite is quite common, and 

 the rock powder yields many grains to the magnet. In some places, however, what 

 appears to be magnetite is probably ilmenite, as it is found partially surrounded by 

 a whitish substance resembling the usual alteration product of ilmenite. 



Three sections examined. 



Age. Probably an eruptive of the Cabotian. 



Eemarks. This rock appears to have been originally a rather coarse-grained 

 diabase. Possibly some orthoclase feldspar may have been present, thus giving the 

 red feldspar of the hand sample, as orthoclase is found to some extent close by in 

 basic rocks (diabase or gabbro), where it is intimately associated with the acid "red 

 rocks" (compare No. IB). In this case, however, it seems possible that the red color 

 of the feldspar is due to staining during the process of decay, and that it was not 

 originally orthoclase. " A cementing material for rounded masses of No. 1."* u. s. G. 



No. 4A. GABBKO. (Breccia, reddened.) 



Duluth. Near the same point as No. ID and No. IE, east of Newson's quarry. 



Ref. Ninth Report, page 11; Tenth Report, pages 41, 99. See also descriptions of Nos. ID, IE, and 1799. 



Meg. The specimen is much reddened, but there is no certainty of any other 

 feldspar than a plagioclase, whose albite twinnings are sometimes visible. It is 

 specked or spotted with red and gray, also with epidotic green, and in general is 

 quite coarse. The specimen marked No. 4A is not a fair representative of the 

 description made in the field, but is rather a coarse sample from the matrix which 

 embraces the undecayed and rounded masses. The origin of this breccia (?) is 

 problematical. 



No section. 



Aye. Cabotian? N. H. w. 



*N.H. WINOHKIJ,. .\inlli Annual lii'jiurt, p. 11. 



