THE LOWER HURONIAN. 345 



and hornblende, witli feldspar present in large quantity. The dark con- 

 stituents predominate, and the mica is usually more abundant than the 

 hornblende. With these constituents there occur varying- amounts of 

 hypersthene, light-green pyroxene, olivine (f), and magnetite. In some of 

 these gabbro contact rocks the hypersthene exceptionally is the predominant 

 constituent, usuallj^ associated with considerable mica and magnetite. In 

 general we may say that the production of minerals rich in magnesium and 

 iron, in particular hypersthene, brown mica, and magnetite, is characteristic 

 of the gabbro contact. These are, of course, the kinds of minerals which, a 

 priori, would be expected in rocks greatly affected by a gabbro magma. 

 Analyses of these rocks and of the rocks from which they were derived 

 have not been obtained, and indeed a great many would be required to 

 prove the thesis that an actual transfer of material from the gabbro to the 

 surrounding sediments had taken place. However, in view of the produc- 

 tion in such large quantity of the magnesian and iron miuQrals in these 

 sediments it is believed that such a transfer of some magnesia and iron has 

 actually taken place from the gabbro magma to the sediments now con- 

 taining these minerals. The fact that these minerals occur more abun- 

 dantly in the rocks near the gabbro than in those farther away supports 

 this view. 



The spotted contact rocks, the spilosites, are known to occur as the 

 result of the contact action of diabases and gabbros, and those occurring 

 in this district are believed to be without doubt the i^roduct of the gabbro 

 contact and to be characteristic of it. The spilosites from the Vermilion 

 district are fairly common in the area southwest of Snowbank Lake. They 

 are very similar to the spilosite described from the Crystal Falls district of 

 Michigan," and, like them, the spots, which are in general of oval outline, 

 occur isolated or united along the long axis of the ovals in a series. These 

 spots are corhposed of aggregates of muscovite, epidote, little chlorite, and 

 sphene, in a fine groundmass (which predominates) of muscovite, chlorite, 

 epidote, sphene, feldspar, and quartz. 



Somewhat different are the spotted rocks occurring north of Paul Lake, 

 for example. The white material forming the spots has essentially the same 

 single and double refraction as feldspar. The material includes biotite and 



«Mon. U. S. Geol. Survey Vol. XXXVI, 1899, pp. 206-207. Also a contribution to the study of 

 contact metamorphism, by J. Morgan Clements: Am. Jour. Sci., 4th series, Vol. VII, 1899, pp. 81-91. 



