284 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



of a single theological Period, who shall say what might not be ac- 

 complished in the course of Eras? 



Important consequences flow from the fact that the goal and ex- 

 pression of most geological research is the construction of a map of 

 the area studied. To the layman who studies a country with a geo- 

 logical nuip in hand, it is hard to resist the conclusion that the ma]) 

 is merely fanciful; he can see no evidence for the lines laid down or 

 the symbols employed, and he is astonished when trenching or drill- 

 ing jjroves their correctness at any particular point. It is diflicult 

 for him to see or to realize the cumulative force of the aggregation 

 of minute pieces of evidence, slight differences in slope or soil, varia- 

 tions in quality, quantity, or luxuriance of vegetation, variations in 

 dryness or moisture, the distribution of culture, the extension into 

 the area of some underlying tectonic plan — the laws of Avhich ma}^ 

 have been worked out elsewhere — and the tliousand-and-one consider- 

 ations which go to make up the mind of the geologist. 



It is, of course, perfectly true that the individuality of the surveyor 

 enters not a little into the extrai)olation of geological lines, beyond 

 the points where direct observation of the rocks is possible. So much 

 is this the case, that it is feasible, from the inspection of his map, to 

 gauge, not only the geological competence, experience, and attain- 

 ments of the survej'^or, but his knowledge and grasp of physiographic 

 form, his power to see into intricate solid geometry, his artistic skill 

 of hand and eye, and, above all, that indefinable (puility liis ".eye for 

 a country " on which so very much depends. 



The construction of a map has the further advantage that it grows 

 by the alternation of periods of observation in the field with periods 

 devoted to the thinking out of structure after each day's work and 

 in the intervals between successive visits to the field, so that with 

 every return to the ground, the facts may be reobserved and lines 

 retested in the light of growing knowledge. It is true that ideal 

 observation should be so complete and exact that icobservation has 

 nothing to teach; but, as a matter of fact, with a map as with a book, 

 what one takes from it is what one brings to it, clarified, improved, 

 and extended. There should be allowed to professional geological 

 surveyors as nuich elasticity as ix)ssible, so that, in addition to de- 

 tailed and exhaustive primary sur\e_y, there may be fretjuent revision 

 in the light of theii- own work and that of their neighbors. In this 

 respect the hand-colored form in which geological uuips were oi'igi- 

 nally published has an advantage over the newer, clieai>er, and more 

 consistent color- printed maps. 



Geologists should give a cordial welcome to the new aid provided 

 by aerial survey and jihotography. This provides the last point of 

 view of their areas, which has been hitherto denied, though they 



