MAPPING OF THE CRYSTALLINE SCHISTS 781 



competent worker who at great expense of labor and thought 

 visits each exposure and studies it with respect to all its neighbors. 



In regions in which rock types are easily distinguished, and 

 where the tectonic structure is simple, the difficulties above 

 referred to are reduced to a minimum ; but in belts in which the 

 rocks have been profoundly metamorphosed, and where oro- 

 graphic forces have brought about a complex structure, the 

 dangers which arise from compounding fact and theory may be 

 so great as to practically destroy the value of the map. 



Outcrop maps needed to display the fact.— The largest 

 element of danger is removed and a map increased very much in 

 value by indicating upon the tinted areas of the different forma- 

 tions (the theory) the position, the nature and the observed 

 peculiarities of the outcroppings (the fact). If a map has been 

 made without sufficient field study this fact will then appear, and 

 if it must be more thoroughly worked out and republished, the 

 earlier work is not lost if it was intelligently and conscientiously 

 done. If, however, it was carelessly, or for any reason in- 

 efficiently done, the examination of a few of the exposures which 

 have been represented upon the map will supply the basis for 

 judgment. In a complex area much time is consumed in making 

 a just estimate of the value of a conventional geological map, 

 and if revision is necessary the work must be taken up de novo, 

 even if the field work was efficient so far as it was carried. 



The serious objection to the general use of outcrop maps is, 

 of course, the great expense which they involve because of the 

 large scale which must be adopted. In the complex region 

 bordering Lake Superior, the United States geological survey has 

 most wisely adopted the plan of publishing outcrop maps to 

 properly present the careful work of the geologists of the division. 

 It is safe to say that the time is not far distant when equally 

 precise methods must be adopted at least by government and 

 state institutions for all regions of equal or of greater complexity. 



FACT.— THE OBSERVATIONS. 



Routine field observations. — Locatio?i of outcrops. — The 

 first requisite for geological mapping in a region of crystalline 



