PRE-CAMBRIAN. 71 



mainly granites, and similarly there are Paleozoic granites in the areas shown as pre-Cambrian. 

 Separation of these minor areas is difficult, even with detailed work, and is not warranted by 

 the present state of our knowledge. It is believed, however, that the present mapping is suffi- 

 ciently accurate to give a fair idea of the distribution of the various igneous and metamorphic 

 rocks of the Piedmont — a matter which has not previously been attempted. The divisions 

 used consist of groupings of formations that have been distinguished in the detailed work in 

 the Appalachian Mountains. . - 1 have described these in full in folios of the United States Geo- 

 logical Survey and my summary of them is given in Survey Bulletin 360 (" Pre-Cambrian geology 

 of North America," by Van Hise and Leith), page 701. 



