The marine Triassic section of .America is unusually complete, and its thickness compares favorably with that of any other region. All three subdivisions-Lower, Middle, and Upper Triassic--are represented by calcareous deposits, aggregating approximately 4,000 feet in thickness. Of this amount, about 800 feet belong to the Lower Triassic, about 1,000 feet to the Middle Triassic, and about 2,000 feet to the Upper Triassic.

The entire section is not represented at any one locality, nor is the thickness of each division constant. Furthermore, the marine Triassic is not everywhere developed as a calcareous formation. In the United States the Triassic system is represented by marine deposits only in the Western States, in Idaho, Nevada, Oregon, and California.

The Lower Triassic is known only in Idaho and southeastern California, where about 800 feet of shales and limestones contain fossils characteristic of this series. The most important genera are Meekoceras, Danubites, Columbites, Flemingites, Aspidites, Lecanites, Ophiceras, Nannites, Ussuria, Pseudosageceras, Hedenstrmia, Cordillerites, Tirolites, and Proptychites, most of which are represented by closely related species in the Lower Triassic of India and Siberia.

The Middle Triassic occurs chiefly in Nevada and southeastern California. In the Inyo Range, southeastern California, about 200 feet of shaly limestones contain the following genera characteristic of the lower horizon of the Middle Triassic: Arochordiceras, Xenodiscus,  Hungarites, Ptychites, Tirolites, Ceratites, and Parapopanoceras.

In central Nevada, in the West Humboldt Range, the higher beds of the Middle Triassic are represented by about 800 feet of shaly limestones, in which the most characteristic genera are Ceratites, Anolcites, Dinarites, Danubites, Celtites, Hungarites, Beyrichites, Arochordiceras, Balatonites, Eutomoceras, Ptychites, Joannites, Lecanites, and Atractites among the cephalopods, Daonella among pelecypods, and Cymbospondyl;us among vertebrates.

The Upper Triassic is best represented in northern California, where the Hosselkus limestone and the shales below and above this formation contain characteristic fossils. The Hosselkus limestone and the interbedded shales have a thickness approximating 500 feet.  Fossils are very abundant throughout this formation, although much better preserved near the base. The best known fauna belongs to the zone of Tropites subbullatus, and contains as its most important members the following genera: Tropites, Paratropites, Discotropites, Juvaites, Sagenites, Leconteia, Trachyceras, Clionites, Arpadites, Polycyclus,  Metairolites, Hauerites, Dieneria, Arcestes, Paraganides, Proclydonautilus, Cosmonautilus, and Atractites among cephalopods, Halobia among pelecypods, and Shastasaurus among the vertebrates.

Above the Hosselkus limestone lie the Pseudomonotis slates, of unknown thickness, characterized by Halorites, Rhabdoceras, Arcestes, and Pseudomonotis.

In the West Humboldt Range in Nevada the Star Peak limestone, about 1,200 feet thick, appears to represent the Hosselkus limestone, although very few fossils have ever been found in it. Above the limestones lie the Pseudomonotis beds, about 800 feet of shales and shaly limestones, in which are found Pseudomonotis subcircularis, Rhabdoceras, Halorites, Placites, and Arcestes, an association characteristic of the Noric horizon.