CHAPTER XXIX 



THE JURASSIC PERIOD 



The name Jurassic was first used by Brogniart and Humboldt, 

 and was taken from the Jura Mountains of Switzerland, where the 

 rocks of this system are admirably displayed. In Europe the 

 Jurassic has long been a favourite subject of study, because of 

 the marvellous wealth of beautifully preserved fossils which it con- 

 tains. For this reason, the Jurassic is known with a fulness of 

 detail, such as has been acquired regarding very few of the other 

 systems ; and no less than thirty well-defined subdivisions have 

 been traced through many countries of the Old World. For this 

 country no such minute subdivision is as yet possible, and only 

 the three primary divisions of the system need be cited. 



Jurassic 

 System. 



^ ,. f Upper Jurassic Series. White Jura, or Malm. 1 

 Oolite ", \ . J T 



I Middle Jurassic Series. Brown Jura, or Dogger. 



Lower Jurassic or Liassic Series. 



Distribution of Jurassic Rocks 



American. — No undoubted Jurassic strata occur in the Atlantic 

 border of the United States, though by some authorities the upper- 

 most part of the estuarine Trias (Newark Series) is referred to 

 this system, and by others the Potomac Series of the Cretaceous 

 is regarded as Jurassic. Whether or not these references be cor- 

 rect, no marine Jura is known on the Atlantic slope of North 

 America except in Mexico : in the latter country are some marine 

 beds which probably belong to this system. In eastern North 

 America the Jura was a time of great denudation, when the high 



1 The Malm is not quite the same as the White Jura, but includes some of the 

 Brown, the remainder of which corresponds to the Dogger. 



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