70 INDEX TO THE STEATIGRAPHY OF NOETH AMERICA. 



sive in the Keewatin, which is restricted, in mapping at least, to areas that have 

 been surveyed in detaU. It was determined to recognize both of these divisions 

 of the Laurentian to the extent that present knowledge justifies such distinction. 



The arrangement of the classifications agreed upon was then taken under 

 consideration and the schedule was corrected as shown in figure 4. The revised 

 classification showing the various usages which are there set forth was then unani- 

 mously adopted on motion of Prof. Chamberlin, seconded by Prof. Adams. 



The classification thus drawn up comprises the usage accepted for this publi- 

 cation by the Canadian Survey and the United States Survey, also the local 

 sequence of eastern Canada, of the Lake Superior region, and of the Cordillera. 

 It presents the presumed correlation of these various classes according to present 

 knowledge and. fixes the responsibility for the mapping of different sections upon that 

 organization which has done most of the work in either district. 



I 16-17, J 17-18. SOUTHERN APPALACHIAN MOUNTAINS. 



The revision of the pre-Cambrian rocks of the Appalachian Mountains by 

 Arthur Keith and the sources of information used by him are stated in the following 

 memorandum, which he prepared for this work. 



The geologic formations of the area between the Blue Ridge and the Coastal Plain and 

 from Maryland to Alabama have been revised from those shown in published maps, according 

 to detailed and reconnaissance work that I have done during the last 20 years. 



In Virginia the boundary at the base of the Cambrian near the Blue Ridge and all bound- 

 aries east of that, except for the Richmond and the Danville Triassic basins, are new. They 

 are based on detailed work as far south as latitude 38° 30'; south of that latitude they are 

 founded upon reconnaissance work. 



In North Carolina the detailed geology of the published folios for the western part of the 

 State is supplemented by reconnaissance in the central part of the State; for the eastern part 

 of the crystalline rocks, the mapping is based on the old map of North Carolina by W. C. Kerr, 

 with some modification and reinterpretation of the areas. 



In the western part of the State also the detailed but yet unpublished work extends from 

 the mountains and the Blue Ridge 20 to 40 miles into the Piedmont Plateau. 



In South Carolina the northwestern margin of the State has been mapped in detail and a 

 detailed survey has been made of the Kings Mountain region ia northern-central South Caro- 

 lina and in North Carolina. The remainder of the mapping in the State is based on the old 

 county maps of Lieber, somewhat modified by my reconnaissance. 



For Georgia manuscript maps of C. W. Hayes have been used west of longitude 84° 30' 

 and north of 33° 30'. My detailed and reconnaissance work affords the basis for mapping 

 formations north and northeast of Atlanta, including the belt of probable Cambrian strata 

 (Brevard schist). Southeast and south of that line I have used the map of the granites' of 

 Georgia published by T. L. Watson, with slight modifications. 



In the rocks southeast of the main Cambrian belt three divisions are made — (1) Paleozoic 

 igneous rocks of post-Cambrian age; (2) undivided pre-Cambrian gneisses and schists; and (3) 

 pre-Cambrian igneous rocks, mainly granites and granitoid rocks. Of the pre-Cambrian igneous 

 rocks, only the granites are mapped in class 3, the Roan gneiss being omitted, although mainly 

 igneous, because most of its areas are too small to show with the present map scale. The 

 effusive pre-Cambrian igneous rocks are included with the granites in the mapping, as no 

 provision is made in the color scheme to separate them. 



Although most of the boundaries shown are those between different pre-Cambrian forma- 

 tions, considerable areas of Paleozoic intrusive rocks are distinguished, especially in Georgia and 

 the CaroUnas. In the areas so shown there are known to be minor areas of pre-Cambrian rocks. 



