272 THE GEOLOGY OP MINNESOTA. 



[Basalt. Porphyryte. 



outlines. The green substance prevailing in the amygdules is opaque between 

 crossed nicols, or nearly opaque, owing to the confused polarization of the crowded, 

 very fine fibres which may be of thalite, or of serpentine. 



Two sections. 



Age. Cabotian ; perhaps pre-gabbro. N. H. w. 



No. 233. BASALT (with oKvine). 



From a dike twenty-one feet wide, near No. 232, horizontally columnar, running N. 15 E., projecting into 

 the bay seventy-five to ninety feet. 



Ref. Annual Report, ix, pages 58, 59. 



Meg. Dark, medium-grained, diabasic. 



Mic. This rock is not ophitic, but granitic, so far as concerns the relation of 

 the augites to the feldspars. The augites are small and not much altered, considering 

 their comparative date of generation. The feldspars are somewhat kaolinized. 

 Magnetite rods, or skeleton crystals are disseminated abundantly, and at the same 

 time magnetite is present in the form of angular and irregular masses, probably a 

 replacement of olivine. There is an abundant remnant of a poorly differentiated 

 magma-glass, through which the magnetite rods are scattered. In these areas are 

 not only incipient minute crystals which are apparently of augite and of feldspar, 

 but also small secretions of quartz, and an isotropic grayish green substance, which 

 represents the final residuum from the glass. 



One section. 



Age. Manitou (?) 



Remark. It is impossible to assign age to the dikes along this part of the shore 

 with any positiveness, since the Manitou may or may not have shared in their 

 production. Considering the fact that the field relation of this rock to No. 234 

 indicates that it is a part of the same mass as that, it exhibits an interesting variation, 

 which requires different names for the two rocks. The glassy remnant seen in this, 

 and the non-porphyritic structure, are contrasted with the holo-crystalline and 

 porphyritic feldspars of the other. N. H. w. 



No. 234. PORPHYRYTE. (Diabase.) 



"From a dike eighteen feet wide running east and west 'hading' a little to the south, cross-columnar, 

 cotemporary and blending with the dike No. 233, the structure of the two running together; of a brownish-black 

 color." 



Ref. Annual Report, ix, page 59. 



Meg. A rather fine-grained, apparently granular, dark, brownish-black rock, 

 with porphyritic plagioclases of about the same color as the mass of the rock. Many 

 of the phenocrysts seem to contain the older materials of the rock poikilitically. 



Mic. The plagioclase phenocrysts shown in the sections are few and small. 

 They are not poikilitic, as is indicated by the hand specimen, only one of them 



