SCIENCE 



NEW YORK, JANUARY fi, 1898. 



THE MICROSCOPE AND THE STUDY OF THE CRYSTAL- 

 LINE SCHISTS. 



BT 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 

 has 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 

 siimmariziog results secured and by striving toward a more pre- 

 cise definition of terms to be employed in descriptions of crystal- 

 line terraces, 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 ap- 

 pears 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 " green- 

 stones" and " greenstone-schists " of the Lake Superior region. 



No problems of geology are more intricate and at the same time 

 1 .ore attractive than those presented by the pre-Cambrian forma- 

 I uri. The stratigraphy, correllation, and genesis of these vast 

 I k masses must be deciphered mostly without the aid of fossils; 

 t ce any kind of evidence, however slight, which throws real 

 • t on the questions at issue must be welcomed by the geologist 

 must be so thoroughly studied by him thait it can be accorded 

 1 ijll significance. 



le sub-division of the pre-Cambrian rocks intodistinot forma- 

 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 means of deciphering a 

 ■crystalline terrane. But the structure planes of the rocks are so 

 often secondary and their original character so obscured by altera- 

 tion, 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 effi- 

 cient than that furnished by the microscope. 



In the history, which in the future will be written of the pre- 

 Cambrian 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 accu- 

 mulating evidence; there repeated surveys have carried on de- 

 tailed mapping and the field study of stratigraphy ; and there, if 

 anywhere, the value of uniting out-of-door and laboratory meth- 

 ods has found demonstration. 



In his present communication, Professor Winchell first sum- 

 marizes the results reached by the Geological Survey of Min- 

 nesota in regard to the classification of various pre-Cambrian for- 

 mations distinguishable within that State. Upon this subject the 

 writer wishes to express no opinion. In the second section of the 



^ The CrystalHue Rocks, some preliminary coDstderations as to their struc- 

 tures and origin.— N. H. Winchell, Twentieth Ann. Report Geol. Survey of 

 Minnesota, 1891. 



paper the use of terms is dealt with. A generally accepted dis- 

 tinction is made between constructive (metamorphic) and de- 

 structive (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 dyna- 

 mic 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 Arch- 

 aean 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 im- 

 plication is that, after all, the ordinary mortal must be satisfied 

 to be either a field, or a microscopical geologist. 



Nort', the writer is not aware that the most ardent advocate of 

 the study of petrography(mlcroscopical or otherwise) considers this 

 branch as more than an aid to geological research. Divorced from 

 tield observation it becomes unreliable and trivial. As a supple- 

 ment 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 micro- 

 scopical study of isolated hand-specimens as mere mineral aggre- 

 gates 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 petrographi- 

 cal 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 ob- 

 tained in the field and laboratory there is no discrepancy, except 

 to one who incompletely comprehends one or the other method 

 of work. 



Professor Winchell says that "the sedimentary structure in a 

 rock is one of those characters which the field geologist only 

 can be allowed to pronounce upon with authority." If this be so, 

 it does not follow that he who is only a field geologist possesses in 

 such cases the greatest authority. If he has microscopical and other 

 petrographical methods to aid him, it stands to reason that his 

 opinion will be worth more. If he is certain in the field, he may, 

 it is true, be brought to doubt by laboratory study, but this doubt 

 is itself a gain, since there are some crystalline rocks whose origin 

 can perhaps never be put beyond doubt. 



Professor Winchell then proceeds to discuss what he calls a 

 concrete case from the greenstones of the Lake Superior region 

 and gives what he thinks would be the conflicting conclusions 

 obtained by a microscopical and field study. To illustrate this 

 case, he reproduces two figures taken from the writer's Bulletin 

 (U. S Geological Survey, No. 63) on the Lake Superior greenstone 

 schists, and says: "These figures couM be repeated many times 

 in the course of a brief examination in the field. These cases 

 present the i-^sues fairly. It remains to be decided whether the 



