486 THE POPULAR SCIENCE MONTHLY. 



together the scattered results of all these in a comprehensive 

 whole, before a beginning could be made on the publication of 

 the geologic map. This work was carried forward for ten years 

 before the first folio of the final geologic map was published. 

 Most of the larger questions affecting the classification and no- 

 menclature of the sedimentary and volcanic rocks were brought 

 to a satisfactory conclusion. Great progress was made in the 

 study of the altered (metamorphic) rocks and of the complex of 

 crystalline rocks grouped under the term "Archean." A satis- 

 factory solution of the lower Mesozoic (Juratrias) series is yet to 

 be reached ; and there is a great field in the pre-Paleozoic sedi- 

 mentary and crystalline formations, in which further study will 

 bring out important data for classification and geologic mapping ; 

 but the areas affected are relatively small within the United 

 States, and the areal geologic mapping can go on without serious 

 injury on this account. Director Powell and members of the sur- 

 vey gave much time and thought as to the best method of repre- 

 senting the geology on maps. The result is a color scheme that, 

 under the skillful application of the editor of geologic maps, has 

 thus far met the demands made upon it. 



The areal geologic map in its final form, as presented in the 

 geologic folios, is intended to place before the geologist, mining 

 engineer, student, and all persons interested the topography and 

 geology of the area included within each atlas sheet. The topo- 

 graphic map has already been mentioned. In each folio a brief 

 explanation is printed of the topographic and geologic maps and 

 of the uses to which they can be applied. It is a simple, short 

 lesson for the layman, to enable him to make use of the folios in- 

 telligently. 



The areal geologic map represents all that the geologist pre- 

 paring it knows of the areal distribution of the rocks occurring 

 within its area, so far as he can delineate such knowledge within 

 the scale of the map. Taken in connection with the topographic 

 base, it presents the geologic distribution of the various rocks in 

 a form for the use of geologists and students, but it does not ap- 

 peal directly to persons interested in the mineral resources o.f the 

 region. To meet this important demand a second map is pre- 

 pared, upon which the rocks carrying minerals of economic value 

 are clearly indicated by distinct colors, the import of which is 

 shown by the colored legend on the margin of the map. Thus the 

 distribution of the coal- and iron-bearing rocks of the Appala- 

 chians in Tennessee and other States, and of the gold-bearing 

 rocks of California and elsewhere, is clearly presented. These 

 maps refer only to the areal distribution of the rocks. What is 

 known of the underground geology is graphically illustrated on 

 a structure section sheet and a sheet of columnar sections. With 



