PREFACE. Xvii 



character and distribution of the different subdivisions of the 

 geological series, as developed on and near the Pacific coast. 



Perhaps the most striking fact we have to present is, the 

 immense development on the western edge of our Continent 

 of rocks equivalent in age to the Upper Trias of the Alps, 

 and palosontologically closely allied to the limestones of 

 Hallstadt and Aussee and the St. Cassian beds, that ex 

 tremely important and highly fossiliferous division of the 

 Alpine Trias. 



This great Triassic belt of the Pacific coast has been most 

 fully explored by the Survey in the latitude of 40, and over^ 

 a width east and west of nearly four degrees of longitude 

 (117 to 121). It is from this region that the largest por 

 tion of the Triassic fossils described in this volume have 

 been obtained, namely, from the three parallel ranges, in lon 

 gitude 117 to 118, in Nevada Territory, known as the 

 Humboldt Mountains, or the Humboldt Mining Region, and 

 from localities in Plumas County, California. But sufficient 

 palscontological evidence has been obtained to enable us to 

 state that this formation extends from Mexico to British 

 Columbia, and that it occupies a vast area, although much 

 broken up, interrupted by eruptive rocks, and covered in 

 many places by heavy accumulations of volcanic materials. 



Among the specimens from the Humboldt ranges, as also 

 from Plumas County and other localities in the Sierra Nevada, 

 Mr. Gabb recognizes at least four species as identical with 

 European, while the whole aspect of our Triassic collections 

 is strikingly like that of the Hallstadt and St. Cassian beds, 

 there being the same remarkable intermixture of Orthocera- 



PAL. VOL. T. P 



