THE KEWEENAW AN. 423 



fresh rocks are basalts or dolerites, and are believed to belong- to the same 

 general period of formation. 



The distribution of these dikes can be seen on the maps in the accom- 

 panying atlas, where those of sufficient size to warrant it have been shown. 

 The majority of the dikes are small and have been omitted in most cases, 

 although a few have been inserted upon the map but are greatly exaggerated. 

 These dikes cross the strike of the slate and other sediments and are also 

 found to run parallel to their bedding. In some of the schistose rocks 

 the dikes very clearly followed the schistosity. 



The dikes can not be said to follow a definite system in their occur- 

 rence, although presumably if they were studied in sufficient detail it would 

 be found that they followed in general the lines of fracture which prevail 

 in those portions of the district in which they occur. 



PETROGRAPHIC CHARACTERS. 



Macroscopic characters. — The rocks are invariably dark colored, black 

 or greenish or brownish black. The rocks in the smaller masses, the 

 narrower dikes, show a very fine grain and could properly be called basalt. 

 The rocks in the centers of the larger dikes show up as coarse-grained 

 ophitic textured rocks and are dolerites. However, gradations between the 

 fine- and coarse-grained forms are shown in that the finer-grained phases 

 appear upon the margins of the large dikes grading up by increasing size of 

 mineral constituents to the coarse-grained dolerites, occupying the centers 

 of the large dikes. The fine-grained basalts seem to predominate. 



Microscopic characters. — Under the microscope, the rocks are found 

 to be very fresh, as one would infer from the fact that they are usually 

 decidedly black. In some few of them slight alterations producing greenish 

 or brownish-green minerals tend to vary these to the brown or greenish 

 tones already referred to. The constituents of the rock are yellowish to 

 violet augite, green hornblende, a feldspar near labradorite in composi- 

 tion, olivine, apatite, ilmenite, and magnetite. These minerals show their 

 normal characters and but rarely give evidence of being altered. Where 

 altered there has been produced chlorite, epidote, calcite, and hornblende. 

 The ophitic texture predominates in these rocks, although the intersertal 

 texture is also common and merges at places into an imperfect fluidal 

 texture brought out by the parallel arrangement of the feldspars. Some of 

 the rocks are porphyritic. 



