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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, peculiarities in the components comprising the classified bodies. The next step, following the use of the microscope in rock investigation, 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 unessential—properties of rocks. After careful work of this kind had finally established the various varieties on the basis of mineralogical 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 classification. It was originally written with a view of emphasizing the distinctive differences between the gabbros and the coarse diabases. 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. 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