On Some Dykes Containing Hukonite. 35 



tion of these phenocrysts of labradorite would produce the so called 

 Huronite. The fine-grained portion of the rock in which these crystals 

 have been developed is a fresh aggregate composed chiefly of plagio- 

 clase (labradorite), augite and olivine. The ophitic or diabasic struc- 

 ture is very pronounced. The plagioclase is usually idiomorphic form- 

 ing an interlacing network of lath-shaped crystals, the interstices ot 

 which are filled with augite and olivine. The augite possesses the red- 

 dish colour and pleochroism so common in diabase, the larger grains 

 showing frequent distortion and occasional dislocation. Both the fel- 

 spar and augite exhibit undulatory extinction as an effect of pressure. 

 The olivine, as usual, occurs in irregular, more or less rounded indi- 

 viduals, only very rarely presenting sharp crystallographic outline. Com- 

 monly, it is rather fresh, showing a colourless or light greenish sec- 

 tion with characteristic high relief, rough surface and brilliant interfer- 

 ence colours. It is rarely so fresh, however, as to be without traversing 

 fissures filled with more or less opaque alteration products. In many 

 instances the original olivine grain is represented by a greenish or yel- 

 lowish material, probably serpentine. Small scales or grains of opaque 

 iron ore (magnetite) are associated with this serpentine indicating that 

 they were also a result of the decomposition of the olivine. Less fre- 

 quently, perhaps, the olivine shows a very interesting and rather un- 

 usual alteration to talc, but the resulting scales of this mineral were so 

 small that this could not be ascertained beyond dispute. The talc is of 

 a very pale green colour, slightly pleochroic, and exhibits very brilliant 

 interference colours between crossed nicois. It occurs as a matted or 

 felted aggregate of very minute scales filling the original olivine grain. 

 The talc is usually accompanied by more or less opaque iron ore and 

 occasionally some chlorite, (i) A considerable quantity of biotite 

 is present which in some cases has undergone considerable 

 ''bleaching" owing to the removal of iron, while in other 

 cases it is altered to chlorite. Apatite is also a tolerably abun- 

 dant accessory constituent. The magnetite occurs usually in irregu- 

 lar black grains, most of which have resulted from the decomposi- 



(i) Vol. III. Geol., Wisconsin, p. 235. 



