THE JURASSIC PERIOD. 61 



at any time since the close of the Pennsylvanian period. Like the 

 eastern part of the continent, the interior was suffering erosion, but 

 since its altitude was probably low, the erosion effected was less con- 

 siderable. The post-Paleozoic, pre-Cretaceous erosion in the interior 

 is less well determined than in the Appalachian belt and the Pied- 

 mont plateau farther east. 



The western part of the continent. — In contrast with the eastern 

 and interior portions of the continent, deposition was in progress in 

 many parts of the west. Along the Pacific coast, the deposition was 

 marine ; in the western interior, in the early part of the period, it was 

 in partially inclosed bodies of water which were sometimes salt, some- 

 times brackish, and sometimes fresh, or in dry basins and valleys. 

 Late in the period, an arm of the sea extended itself over a great area 

 of the western interior (see Fig. 346). For convenience, the terms 

 Lower, Middle, and Upper Jurassic are here used in connection with 

 the system in the west, though they are not in general use in North 

 America. 



The Lower and Middle Jurassic of the Pacific coast. — During the 

 epoch represented by the Lower Jurassic beds, corresponding in a 

 general way with the Lias of Europe, marine deposition was taking 

 place on the Pacific coast 1 (California and Oregon) west of the Basin 

 land. Much of the Jurassic of the coastal belt is concealed beneath 

 igneous rock of later origin, so that its original extent is not known. 

 In the latitude of Nevada and Utah, it extended east to longitude 117°. 

 Where both are present, the Lower Jurassic beds generally rest on 

 the Trias conformably, though the younger beds overlap the older 

 system at some points, and fall short of it at others, and locally (some 

 points in the Sierras) there is unconformity between them. The 

 deposits of the Lower Jurassic embrace all the usual sorts of sedi- 

 mentary rocks. Beds of corresponding age are not known in British 

 Columbia, and this part of the coastal belt was probably land, and 

 suffering erosion. 2 Early Jurassic beds occur in some of the islands 



1 For the Jurassic of the Pacific coast, see Hyatt, Bull. Geol. Soc. of Am., Vol. Ill, 

 and Vol. V, both articles chiefly paleontological; Meek, Paleontology of California, 

 Vol. I, and the following folios of the IT. S. Geol. Surv.: Bidwell Bar, Colfax, Downie- 

 ville, Jackson, Lassen Peak, Maryville, Mother Lode, Nevada City, Pyramid Peak, 

 San Luis, Sonora, Truckee, Cal., and Roseburg, Ore. 



2 Dawson, Science, March 15, 1901. 



