GEOLOGIC DIVISIONS MAPPED. 17 



Most of the Paleozoic in New England is represented as undivided, because the 

 metamorphism which the rocks have suffered has so far prevented recognition of 

 divisions. In northwestern Maine it is rather the drift covering and the monotony 

 of the black slates of the several terranes distinguished in Canada that has interfered 

 with mapping. 



In the northern Cordillera from Idaho to Alaska the unknown or little-known 

 strata are generally classed as Paleozoic, but the few areas that have been worked 

 up in detail indicate that the region really is a fine mosaic of many formations. 



In the West Indies and in Venezuela the pre-Mesozoic sediments are classed 

 as Paleozoic. 



The systems of the lower Paleozoic (Cambrian and Ordovician) can not be 

 mapped on this scale as separate terranes divided by the boundary which paleon- 

 tologists have generally recognized, for the line in many areas falls in a great thick- 

 ness of limestone which is not readUy divisible. It has been found practicable to 

 distinguish Cambrian and lower Ordovician from middle and upper Ordovician, 

 the former including all below the Chazy and its equivalents, whereas the latter 

 takes in the Chazy and all later Ordovician terranes. 



The Helderberg of the New York and eastern Canada sections is classed as 

 Devonian in the text and in the legend but is included with Silurian on the map. 

 The difference arises from the facts that the Geological Survey of Canada has not 

 yet recognized the Devonian age of the Helderberg and that the United States 

 Geological Survey has only recently done so. The map would in most places not 

 be perceptibly changed, however, by shifting the lines to accord with the text. 



In the eastern United States the Carboniferous is readily divisible and the 

 divisions (Mississippian, Pennsylvanian, and Permian) are easily shown upon the 

 map. In western North America the Carboniferous has a different development 

 and the areas are, moreover, too small to be subdivided on the scale of this map. 

 Thus, although the equivalents of the Mississippian and Pennsylvanian are known 

 in western North America, they are not separated on the map. 



The Permian is not yet clearly distinguished from the older Carboniferous in 

 some areas or from the Triassic in others. The adjustment of uncertainties can 

 best be followed by referring to the authors quoted in the chapters on the Car- 

 boniferous and Triassic. 



The Triassic of the Arctic is mapped according to Schei and extended to the 

 Cape Rawson beds, which lie in the strike. 



The Jurassic occupies but very small areas. Although it is widely represented 

 in the central Rocky Mountains and thence northward, it occurs in such narrow 

 belts that it can not be mapped separately from the Triassic in that province. It 

 was not possible to insert another band of color between those representing the 

 Triassic and Cretaceous. 



The line between Jurassic and Cretaceous in the Coast Range of Cahfornia is 

 drawn by different observers below the Franciscan, at the base of the Knoxville, 

 or in the upper part of the Knoxville. The mappmg expresses the line at the base 

 of the Knoxville, the only one that can be readUy traced. 



48011°— 12 2 



