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THE MICROSCOPE AND THE STUDY OF THE CRYSTALLINE SCHISTS.

BY GEORGE H. WILLIAMS, JOHNS HOPKINS UNIVERSITY, BALTIMORE, MD. IN some preliminary pages from the Twentieth Annual Report of the Geological Survey of Minnesota, Professor N. H. Winchell bas recently circulated some considerations on the structures and origin of the crystalline rocks. In so far as these are the expres sion of a sincere desire to advance this difficult line of inquiry by summarizing results secured and by striving toward a more precise definition of terms to be employed in descriptions of crystalline terranes, they are worthy of appreciative consideration by all geologists. Certain of Professor Winchell's statements relative to the comparative value of microscopical and field evidence seem, however, liable to cause misapprehension, and it therefore appears to the writer worth while to call attention to these, at least in so far as they involve his own work on the so-called "greenstones" and "greenstone-schists" of the Lake Superior region.

No problems of geology are more intricate and at the same time more attractive than those presented by the pre-Cambrian formations. The stratigraphy, correllation, and genesis of these vast rock masses must be deciphered mostly without the aid of fossils; hence any kind of evidence, however slight, which throws real light on the questions at issue must be welcomed by the geologist and must be so thoroughly studied by him that it can be accorded its full significance.

The sub-division of the pre-Cambrian rocks into distinct formations has long been recognized as a desideratum in geology but one unattainable without minute and detailed work. General theories have proved futile for its accomplishment. Only now has the problem begun to be attacked by methods which are a stimulus for the present and a promise for the future. In Great Britain, Germany, Norway, Russia, Canada, and the United States facts are being rapidly gathered whose ultimate correlation will surely bring order out of chaos. Field study, areal mapping on a large scale, and the detailed study of stratigraphy must always be the first and most important mears of deciphering a crystalline terrane. But the structure planes of the rocks are so often secondary and their original character so obscured by alteration, that stratigraphy, and indeed all field evidence, may prove inadequate to the task set for it. Then it is that help from other sources is required, and none has thus far shown itself more efficient than that furnished by the microscope.

In the history, which in the future will be written of the preCambrian formations, the work already accomplished in the Lake Superior region must occupy a most honorable place. Many pioneers have there pointed out methods and secured results which the world will recognize as fundamental. There the large number of workers have stimulated discussion and has led to a constant re-examination of the same points in the light of accumulating evidence; there repeated surveys have carried on detailed mapping and the field study of stratigraphy; and there, if anywhere, the value of uniting out-of-door and laboratory methods has found demonstration.

In his present communication, Professor Winchell first summarizes the results reached by the Geological Survey of Minnesota in regard to the classification of various pre-Cambrian formations distinguishable within that State. Upon this subject the writer wishes to express no opinion. In the second section of the

1 The Crystalline Rocks, some preliminary considerations as to their structures and origin.-N. H. Winchell, Twentieth Ann. Report Geol. Survey of Minnesota, 1891.

paper the use of terms is dealt with. A generally accepted distinction is made between constructive (metamorphic) and destructive (weathering) processes of rock alteration, and a plea is entered for some "middle ground” between the interpretations given to the various parallel structures in crystalline schists by those who hold too exclusively to either a sedimentary or a dynamic theory of their origin.

In the third division of his paper Professor Winchell discusses the comparative value of microscopic and field evidence, and it is here that the writer would take issue with his conclusions. He says: "It is in the nature of the problem involved in the study of the complicated structures and relations of some of the Archæan rocks, that the differences between the microscopic evidence and that derived from their macro-structure shall gradually fade out and that one or the other shall usurp the whole field." Later he does indeed allow that "this is not intended to shut out any individual geologist from exercising the right to employ any and all lines of research for the solution of all the problems that he has to solve," (!) but in spite of this generous permission the implication is that, after all, the ordinary mortal must be satisfied to be either a field, or a microscopical geologist.

Now, the writer is not aware that the most ardent advocate of the study of petrography (microscopical or otherwise) considers this branch as more than an aid to geological research. Divorced from field observation it becomes unreliable and trivial. As a supplement to field-work it is most serviceable, as the beautiful results of Iddings, Cross, Van Hise, and many others in this country (not to mention European investigators) fully show. The microscopical study of isolated hand-specimens as mere mineral aggregates once served a useful purpose, but this stage in petrography has now passed.

If, then, it be the acknowledged duty of every petrologist to be at the same time a field geologist, and to study his material in the laboratory in the light of his own observations in the field, is it at the same time too much to expect that the field geologists at work on the crystalline rocks will thoroughly inform themselves of the methods, progress, and aims of petrographical research, at least before they complain of their tendency to mislead? The microscope is now but one of the elements in modern petrographical investigation. Progress made by many workers is constantly advancing the point of view, as well as multiplying methods. Is it fair that the field geologist should remain more one-sided than the petrologist would allow himself to be? Between results obtained in the field and laboratory there is no discrepancy, except to one who incompletely comprehends one or the other method of work.