PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 135 



Diabase.] 



pendicular to each other, in which extinction takes place parallel to the cleavage 

 cracks. One cleavage was distinct, even marked, and the other was imperfect. 

 These grains, therefore, unless they be of augite cut in the zone of symmetry, must 

 be of olivine, cut in the zone 100:010, and the cleavages those parallel to the prism 

 and to the base 001. The oldest olivines, which were numerous, are wholly changed 

 to magnetite and the usual green products. 



Magnetite is not only in the fresh and undecayed pyroxenes, but also in the 

 feldspars, though less commonly. It is most abundant in connection with" the 

 greenish grains which are now chloritic, and in this situation it is very often the 

 result of a change from other substances, probably, in the main, from the older 

 olivines whose forms are outlined by the chloritic areas. Although a part of the 

 magnetite seems to date from the magmatic state of the rock, no distinct cubic out- 

 lines are perceptible. 



Apatite needles pierce the feldspars in great numbers and are in the other minerals. 



Chlorite masses are common and seem to have the form of the olivinitic grains 

 of the earliest generation. 



Quartz is present in scattered grains, so sharply separable from the surrounding 

 minerals that it has the appearance of being an original secretion from the magma. 

 Were the rock much decayed this quartz might be taken for a secondary product; 

 but the rock is quite fresh. 



One section examined. 



Age. Cabotian. 



Remarks. Another section of this rock, made later, brings out some characters 

 not before distinguishable, viz. : some of the green " chloritic " masses are green horn- 

 blende instead of chlorite. These hornblendes are derived from or accompanied by 

 a chloritization apparently of the early olivines. Through them run the magnetite 

 accumulations, marking the irregular cleavages of olivine, and these are also seen in 

 the chlorite areas, though here they do not follow so exactly the old cleavages. The 

 hornblende cannot be said, correctly, to have epigenized on the chlorite, although 

 these two are sometimes found in the same grain so related as to suggest that origin; 

 but probably they are nearly cotemporary growths after olivine. In a single case 

 a brown hornblende forms a part of a border of an augite. 



Brown remnants of the glassy magma are also discernible. This rock is so 

 strikingly like the great dikes at Grand Portage bay (Nos. 248, 253) that it seems 

 necessary to classify it with them. Its structure and composition even conspire to 

 warrant the designation hornblende gabbro. It is hence of Cabotian age, and an 

 eruptive that preceded the Manitou. This is evidently a heavy surface flow allied 

 to the Beaver Bay diabase but of later date, or a sill in the Animikie now denuded. 



N. H. w. 



