12  LETTER    OF    TRANSMITTAL. 
based  on  physical  characters  and  relations,  and  mnch  attention  is 
therefore  given  to  the  discussion  of  the  possibilities  and  limitations  of 
correlation  by  means  of  physical  data.  It  is  concluded  that  with  pres- 
ent information  the  correlation  of  pre- Cambrian  series  and  formations 
of  different  geologic  provinces  is  impracticable. 
Very  respectfully,  your  obedient  servant, 
G.  K.  Gilbert, 
Geologist  in  charge. 
Hon.  J.  W.  Powell, 
Director  U.  S.  Geological  Survey, 
