THE MAKING OF THE GEOLOGICAL TIME-SCALE. 35 



in North America the great part of the eastern half of the 

 United States was raised permanently above water, and is 

 therefore, except at its margins, devoid of records of later 

 marine life. A border of a few hundred miles on the east and 

 south contains Mesozoic and Cenozoic deposits and their 

 characteristic fossils ; but the larger part of the areas covered 

 by Triassic, Jurassic, and Cretaceous deposits are found west 

 of the 97th meridian, or of a line running from the western 

 border of Minnesota to the western shore of the Gulf of 

 Mexico. The disturbances recorded in this elevation of the 

 quarter of a continent were felt in other parts of the world, 

 and occasioned great shifting of marine conditions of environ- 

 ment, causing migration or extinction of great numbers of 

 organisms, and opening up new regions with new conditions 

 to which the organisms of the Mesozoic were rapidly ad- 

 justed. 



Triassic System. The TRIASSIC SYSTEM was first defined 

 by Alberti in 1834.* The rocks which were grouped together 

 to constitute the Trias system are the Bunter sandstone, 

 overlaid by a middle calcareous 'member, the Muschelkalk, 

 followed by sandy shales, and the Keuper; and they are well 

 represented in central and southern Germany. 



This system is poorly represented in eastern America so 

 poorly that the United States Geological Survey proposes to 

 join the Trias and Jurassic of America into a common group, 

 calling it the Jura-Trias system, distinguished by a common 

 continuous fauna and flora. In the Rocky Mountain region 

 there are thick deposits, mainly sandstones, with few fossils, 

 which are intermediate between the Permian, or closing for- 

 mation of the Paleozoic age, and the Cretaceous formations ; 

 but it is difficult to determine in particular cases whether the 

 rocks should be classed with the European Triassic or Jurassic 

 systems. In California Dillerf has recently described fossi- 

 liferous Triassic terranes containing typical Triassic marine 

 faunas. 



* " Beitrag zu einer Monographic des Bunter sandsteines, Muschelkalkes, 

 und Keupers," Stuttgart u. Tubingen, 1834. 



f " Geology of the Taylorville Region of California." Bull. Geol. Soc. 

 Am., vol. HI. pp. 369-394, July, 1892. 



