460 SCIENCE PROGRESS. 



An original parallel-structure or banding is found in some 

 of the rocks, and cataclastic structures are frequent. When 

 iron-ore is present, it is always highly titaniferous, and in 

 places there are in the anorthosite masses of ilmenite with 

 only subordinate olivine and plagioclase. 



The similar and perhaps coeval rocks of the Lake 

 Superior region have been investigated by Lawson (2) on 

 the Minnesota coast. They are fresh, coarse-textured, 

 massive rocks composed almost wholly of felspar. This 

 has been stated by Winchell to be labradorite, while 

 Irving described it as anorthite. Lawson finds both these 

 varieties in the rocks of different localities. The only other 

 constituent present is a little augite, partly in grains, partly 

 in minute parallel inclusions in the felspar, and the rocks 

 are thus of remarkably simple mineralogical constitution. 



Bayley has undertaken a general study of the basic 

 massive rocks of the Lake Superior region (3). The most 

 important instalment of his results deals with the great 

 eabbro mass at the base of the Keweenaw formation in 

 North-eastern Minnesota. This has sometimes been 

 regarded as a "flow," but he suggests that it is either a 

 great intrusion in the lower part of the Keweenaw or an 

 older eroded massive rock, upon which the latter has been 

 deposited. As the rock has the characteristic structure of 

 a plutonic mass, it is indeed difficult to believe that it can 

 have consolidated under superficial conditions. Its con- 

 stituent minerals are felspar, augite, olivine, and magnetite,, 

 and variations in the relative proportions of these give rise 

 to different types, very rich in one or other of these con- 

 stituents. A point of some theoretical importance is that 

 these constituents seem to be the same in rocks which 

 differ widely in total composition. The felspar is always 

 a basic labradorite ; the olivine is a hyalosoderite with 34 

 per cent, of ferrous oxide ; the magnetite is not titaniferous 

 in the normal rock, and the aggregates of titaniferous iron- 

 ore well known in Minnesota belong to rocks differing 

 in various respects from the great mass. 



Near the base of the main mass of gabbro, along its 

 northern edge, occur a number of partially banded rocks of 



