THE PETROGRAPHIC GEOLOGY OF THE CRYSTALLINE 

 ROCKS OF MINNESOTA. 



BY N. H. WINCHELL AND U. S. GRANT. 



The following work is in keeping with the general plan which was adopted 

 several years ago. The field numbers of the specimens are preserved. These numbers 

 have been used to identify them in the various annual reports and in the several 

 bulletins. The specimens, preserved in the university, will serve for many years to 

 verify or correct the conclusions to which we have arrived. The intention has been 

 to describe, with at least a classificatory designation, but usually with some exactness, 

 and by means of microscopical examination of thin sections, every rock specimen 

 that has been collected and reported with a field number in the annual reports, to 

 which the student is referred for their field relations. In other places these litho- 

 logical determinations are employed in the discussion of the systematic and areal 

 geology, and in those chapters many new field observations will be found. 



We have adopted the descriptions of other geologists whenever they have 

 been sufficiently full and have served our purpose; but when our specimens did not 

 answer to the descriptions published by others, or there was some doubt as to the 

 identity of locality, we have made our own descriptions. It will be found that many 

 of the field descriptions require correction, and that too, in some cases, when such 

 descriptions were supplemented by some laboratory examination before publication. 

 In some cases, also, some of the field numbers will be found missing from this 

 enumeration. That is for the reason that the rocks represented were found to be of 

 little importance or they were of doubtful relationship with the others, or because 

 they are not found now in the collection. 



Habitually we have given the megascopic and microscopic characters separately, 

 and have made use of chemical analyses whenever possible, many of which are new. 



The work is divided into two parts, viz.: Part II, which embraces all special 

 petrographic facts, microscopic and descriptive, and Part III, which embraces such 

 discussions and comparisons as to genesis and relationship as appeared to be the 

 result of the foregoing, or to be germane to the petroyraphic yeoloyy of the crystalline 

 i-iH'L-K of tlif state. This, therefore, does not include the descriptive geology, which 

 composes vol. iv, nor the systematic and structural geology which is given in Part I 

 of this volume. 



