434 THE JOURNAL OF GEOLOGY. 



slight differences noted in the characteristics of their components. 

 The number of such groups became larger and larger until 

 eventually there were almost as many sub-groups recognized as 

 there were students who had investigated them. Thus the 

 classification grew complicated, because the criteria upon which 

 it was based were mainly unessential, though prominent, pecu- 

 liarities in the components comprising the classified bodies. The 

 next step, following the use of the microscope in rock investi- 

 gation, consisted in the consolidation of several sub-groups into 

 one larger group — a result due directly to the comparative ease 

 with which the microscope enables the student to distinguish 

 between the primary and secondary — the essential and unessen- 

 tial — properties of rocks. After careful work of this kind had 

 finally established the various varieties on the basis of mineral- 

 ogical composition, attention was directed to the manner in 

 which the rock components are associated — to the rock structure 

 — and an explanation of variation in structure was sought in the 

 environment of rock masses. The study of the gabbros thus 

 became a geological study rather than a mineralogical one. 



The brief historical sketch of the classification of the 

 granular basic rocks, with special reference to the differentiation 

 of the gabbros from the remainder of the group, will thus serve 

 to illustrate the successive steps with which rock classification in 

 general has progressed. But the sketch is not offered here 

 solely as an illustration of the development of rock classifica- 

 tion. It was originally written with a view of emphasizing the 

 distinctive differences between the gabbros and the coarse dia- 

 bases. In the Lake Superior region there exist many coarse 

 basic rocks that have been called indiscriminately " gabbros." 

 Some of these possess the features of true gabbros, as defined 

 by a study of the history of this group of rocks, and others the 

 peculiarities of diabases. Until the distinction between these 

 two types is clearly recognized, it will be impossible to discuss 

 the causes of their differences. It is hoped that the present 

 contribution will serve partly to clear the ground for a careful 

 study of the coarse basic eruptive rocks of the Lake Superior 



