80 JURASSIC SYSTEM. 



CHAPTER XXXVI. 



JURASSIC SYSTEM. 



§ 174. The Jurassic System was named from the Jura Mountains, of Switzer- 

 land. No Trigonia, Belemnites, Ammonites, or specially characteristic fossils of the 

 Jurassic, have been found on the Atlantic side of the continent, notwithstanding 

 the upper part of the rocks described in the last chapter may be Jurassic. The 

 Jurassic fossils, however, occur in the Rocky Mountain Ranges from Mexico to 

 the Arctic regions. The rocks exist in every State and Territory throughout that 

 vast extent of country, varying in thickness from a few hundred feet to 10,000 

 feet. They follow the Triassic, and generally rest upon it. Fossils have been de- 

 scribed from California, Arizona, New Mexico, Idaho, Colorado, Nevada, Montana' 

 Dakota, British Columbia, Cook's Inlet, Alaska, Point Wilkie on Prince Patrick's 

 Land, and the islands north of Griunell Land. In some parts of its grand geo- 

 graphical distribution it is composed of sandstones and clays, resembling, in appear- 

 auce, the Triassic ; but in others it consists of limestones, sandstones, shahs, and 

 clays, indicating shallow water, and bearing no resemblance to the Triassic. The 

 limestones are frequently fossiliferous, and show the progress animal life had made 

 in the ocean, and vegetation had made on the land. Of 50 genera of vertebrates 

 described from the Jurassic, none of them are Paheozoic, and only two have been 

 doubtfully identified in the Cretaceous. Ammonites, Ceratih*, and Belemnitet made 

 their first appearance in the Jurassic, and became extinct in the Cretaceous The 

 genus Spirifera, so abundant in the Devonian and Carboniferous, became extinct in 

 the Jurassic. Several genera of mammalian remains have been defined from the 

 Jurassic, but they are all peculiar to it. No single species of plant or animal is 

 common to the Jurassic and any other formation. Ten genera of Carboniferous 

 plants have been identified in the Jurassic, and four genera occurring in the .Juras- 

 sic have been identified in the Cretaceous. There is a general progress among the 

 invertebrates toward succeeding ages, but the evolution of the vertebrates is very 

 much more marked. There is almost universal unconformability with the overlying 

 Cretaceous, and hence there is an era of time not represented by the rocks. It has 

 been called the Reptilian age, because of the gigantic saurians which then infested 

 the seas. Some of the rocks belonging to this System in California, and, especially 

 about Mariposa, are said to be gold-bearing, but minerals are generally very scarce. 



