MAPPING OF THE CRYSTALLINE SCHISTS 789 



THEORY.— THE DRAWING OF BOUNDARIES AND COLORING 



OF MAP. 



The canons of geological mapping for the crystalline areas. 

 — The determination of what types of rock shall be grouped under 

 one formation-color may be a problem of the greatest difficulty, 

 but in the absence of conglomerates or clearly marked uncon- 

 formities, the question is a petrological one to be settled by the 

 best judgment of the observer on the ground checked by subse- 

 quent microsopical and perhaps chemical investigation in the 

 laboratory. Where the contacts are not sharp but exhibit 

 gradational facies, there may be a question as to where the divid- 

 ing surface should be fixed, but this is after all a matter of 

 comparatively small moment, and however determined, there is 

 little danger that much violence will be done the facts. 



The drawing of boundaries where contacts are not exposed — 

 and contacts are generally covered — is a problem calling not 

 only for much judgment but often also for a large imagina- 

 tion. In most domains of science the worker is allowed to 

 express his lack of adequate data upon which to base a conclu- 

 sion, and to reserve his judgment pending the obtaining of fuller 

 information. Here, however, the demand is peremptory and the 

 line must be drawn with or without knowledge. It is no uncom- 

 mon experience to find within a region of intricate geological 

 structure an alluvial valley a mile or more in width bordered 

 by drift mantles of only lesser width before the bed rock is 

 exposed. However the boundary lines of the formations are 

 drawn, the map maker may be reasonably sure of one fact only, 

 — that the reality is totally different from the representation. 

 Convention requires that all areas shall be colored to show the 

 underlying rock, and it may at least be said that the result is 

 every whit as attractive and plausible in appearance as though 

 the coloring were based throughout upon observed facts. 



It is perhaps a step in the right direction to leave the lakes 

 and rivers uncolored, as is now done by the United States 

 geological survey upon its atlas sheets; but to this also there 

 are serious objections, as, for example, in the area about New 

 York city, where the numerous bridges, piers and abutments, tun- 



