THE 



JOURNAL OF GEOLOGY 



JULY-AUGUST, 1893. 



THE BASIC MASSIVE ROCKS OF THE LAKE 

 SUPERIOR REGION. 



Introduction. — Before the application of the microscope as a 

 geological instrument the classification of rocks was dependent 

 largely upon their apparent similarities and dissimilarities as 

 noted by the unaided eye. When the use of this instrument 

 became almost universal it was found that many rock types 

 similar macroscopically were very different from each other in 

 microscopic appearance, and very dissimilar genetically, while 

 many of the apparently dissimilar types were discovered to owe 

 their differences in appearance simply to the ordinary processes 

 of weathering, which masked their original essential character- 

 istics with the products of mineral alteration. 



The rocks now known as gabbro are quite well characterized 

 by peculiarities that are strikingly uniform in their essential 

 features, though formerly the term was made to cover a large 

 number of closely related but quite different rock types. Their 

 history affords a good illustration of the manner in which rock 

 classification developed from its early independent form to its 

 present highly differentiated but well defined one. 



In the case of the gabbros, as well as in the case of other 

 rock groups, there were at first included under one name all rocks 

 whose superficial features were similar to those of the type 

 originally described. Later, more discriminating study separated 

 this group into a large number of subordinate groups, based on 



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