278 THE GEOLOGY OP MINNESOTA. 



[Gabl.ro. 



Mr//. Porphyritic, with a glassy, clear plagioclase, hard and massive, but fresh 

 and rather light colored, owing to the general effect of the feldspars when fractured. 

 The matrix is dark and medium grained. 



Mic. The feldspar has an extinction on 010 of 25, indicating labradorite. It 

 is abundant in the rock, and was formed after the augite or cotemporary with it. It 

 is clear and brilliant, but slightly zoned. It shows occasional Baveno and Carlsbad 

 twinning, in addition to the albite and pericline types. 



The augite is markedly translucent and clear, but not colorless. It has strong 

 prismatic cleavage, but is not diallagic. Its forms are in part independent of the 

 feldspar, and the crystals are not large. They are well preserved, considering the 

 fact that adjoining them are quite commonly seen hornblendic areas which are 

 usually considered the result of alteration of augite. 



Olivine is in the rock in small amount, serpentinized in the cleavages, embracing 

 the feldspars. Quartz also is not uncommon. It is usually found in the form of 

 small, angular grains embraced in or near those areas that show considerable alter- 

 ation as from undifferentiated magma, accompanied by brownish or greenish and by 

 opaque substances. 



Hornblende is not common, absorptive and yellowish green to brown in natural 

 light. In its immediate vicinity are sometimes small areas of chlorite, which seem 

 to be the result of further alteration. 



Glass, or glassy remnants from the original magma, seem to have been the 

 instigating cause of all these changes. It appears that, perhaps during the cooling 

 stage after solidification, the remaining portions of the magma were the seat of 

 further differentiation, and that the resultant crystals were hornblende, quartz and 

 chlorite. 



Apatite spicules are very abundant. They pierce all the other minerals, and 

 seem to have been the oldest secretion from the molten magma. 



Magnetite is present, but not in considerable amount. Another section shows 

 more magnetite. 



Two sections. 



Age. Manitou(?) 



Remark. This is the most westerly known point on the lake shore where this 

 rock, which seems to be a representative of the Grand Portage Manitou dike series, 

 occurs in sight. These dikes constitute a mountain range lying north from the Saw- 

 teeth range, and distant from the latter at Double bay from three to six miles. Still, 

 notwithstanding the classification adopted, it is impossible to separate the Cabotian 

 dikes at Grand Portage from those seen at Double bay, and at Deronda and Horse- 

 shoe bays. 



