710 THE JOURNAL OF GEOLOGY. 



Camerlander, 1 in 1887, described a similar intergrowth of these 

 two minerals around the garnets of a contact rock from Prachatitz, 

 in the Bohemian Forest, and mentioned that it strongly resembled 

 the kelyphite rims around garnets in serpentine. 2 



Biotite is present in many sections of the gabbro, though not 

 in all. It not only occurs in the neighborhood of magnetite, 

 where this mineral is in contact with plagioclase, but it is some- 

 times found imbedded in the feldspar and augite, and at other 

 times it forms a mosaic with decomposed diallage. In basal sec- 

 tions it is reddish brown, and in longitudinal sections is light 

 yellow normal to the cleavage, and dark brownish-green, almost 

 opaque, parallel to this structural feature. In all cases it is prob- 

 ably secondary, for, even when it apparently occurs alone, a very 

 close inspection of its sections will often reveal remnants of mag- 

 netite grains imbedded in it. This form of the mineral is evidently 

 a reaction product between the magnetite and the plagioclase by 

 which it is surrounded. The remainder of the mica is probably 

 derived mainly from diallage, since when this mineral is perfectly 

 fresh biotite is absent from the rock, and when the pyroxene has 

 undergone any kind of decomposition, little flakes of biotite are 

 intimately intermingled with its undoubted alteration products. 

 In the broad pieces of diallage in which the dark platy inclusions 

 are so common, little flakes and tiny needles of biotite are fre- 

 quently discovered lining the cleavage cracks, so that such pieces 

 not uncommonly are crossed by two sets of inclusions cutting 

 each other at some acute angle, one set comprising the gabbroitic 

 kinds already described, and the other set the biotite plates along 

 the cleavage cracks. 



Magnetite is widespread throughout the rock, but it is not 

 abundant in most sections. It is in small grains, and in tolerably 

 large areas that are broadly rod-shaped or very irregular in out- 

 line. In most cases it occurs between neighboring plagioclase 



z Jahrb. d. K. K. geol. Reichsanst, 37, 1887, p. 117. 



2 The writer is informed by Dr. J. J. Sederholm that intergrowths similar to those 

 occurring in this Minnesota rock are common in Norwegian gabbros and in one from 

 Ylivilska, in Finland. In his university lectures Professor Brogger calls them "coron- 

 ites." 



