806 C. H. GORDON 
GENERAL RELATIONS OF THE PENNSYLVANIAN ROCKS IN THE REGION 
In the Rio Grande Valley the Upper Carboniferous or Pennsyl- 
vanian series is represented by a succession of limestone, shale, and 
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sandstone formations, having an aggregate thickness of from 3,000 
to 5,000 feet. These may be separated into two main divisions or 
groups, to the lower of which the name Magdalena is here applied, 
from the Magdalena Mountains, where they are well exposed, while 
the upper division comprises the formations, chiefly sandstone, to 
