CHAPTER XV 

 THE JURASSIC PERIOD 



Origin of Name, Subdivisions, etc. 



The rocks of Jurassic age are of peculiar interest because they 

 comprise one of the very first systems whose subdivisions were 

 carefully determined by the use of fossils, this work having been 

 done in England about one hundred years ago by William Smith, 

 who is often called the father of historical geology. Smith applied 

 the name "Oolitic" to the system because of the common occur- 

 rence of so much oolitic limestone, but this term later gave way to 

 the term "Jurassic," so-called from the Jura Mountains, between 

 France and Switzerland, where the rocks of the system are un- 

 usually well exhibited and have been much studied. In Germany, 

 too, much study has been devoted to this system, largely because 

 of the abundance of well-preserved and interesting fossils. 



In western North America, where the only undoubted Jurassic 

 strata occur on this continent, the subdivisions of the system are 

 not so well known and correlated, so that various more or less local 

 formation names are still employed. The following table gives a 

 summary of the principal subdivisions for three important regions: 



Pacific Coast of 

 North America 



England 



Germany 



f Upper Jurassic. 

 Jurassic 1 ,,.,,, T 

 System 1 Middle Jurassic. 



( Lower Jurassic. 



Upper Oolite. 



Middle Oolite. 

 f Lower Oolite. 

 \ Lias. 



Malm (or White Jura). 

 Dogger (or Brown Jura). 

 Lias (or Black Jura). 



Distribution and Character of the Rocks 



General Distribution. — Except in Mexico and western Texas, 

 rocks of undoubted Jurassic age are wholly confined to the western 



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