No undoubted Jurassic strata occur in the Atlantic border of the United States, though by some authorities the uppermost part of the Trias {Newark Series) is referred to this system, and by others the Potomac Series of the Cretaceous is regarded as Jurassic, at least the lowest portion of it. Whether or not these references be correct, no marine Jura is known on the Atlantic slope of North America except in Mexico. In eastern North America the Jura was a time of great denudation, when the high ranges of the Appalachian Mountains were much denuded and the newly upheaved, tilted, and faulted beds of the Trias were wasted and probably worn down to a peneplain.

At the beginning of the period Mexico was generally elevated, causing the sea to withdraw from those areas, as yet of unknown extent, which it had covered with shallow water in the Upper Triassic. At the same time the southeastern portion of the country was invaded by the sea and would seem to have remained submerged during the Lower and Middle Jurassic, but at this time most of Mexico was land, and denudation swept away most of the marine Triassic, leaving but a single known area.

In the western Interior region, what is believed to be Jurassic is, for the most part, placed there on stratigraphical grounds only, for few of the rocks have yielded fossils. The beds m question are largely sandstones which in many places rest upon Triassic strata and extend from northern Utah southward into Arizona and New Mexico, and westward from the Rocky Mountains to the Great Basin land which bordered the Pacific. Whether these doubtful beds represent the whole Jura or only a part and, if so, what part, are questions that cannot yet be answered. Some of the sandstones contain gypsum, testifying to salt-lake conditions.

On the Pacific coast the Lias occurs in California, Nevada, and Oregon, but not in British Columbia, and recurs in Alaska and some of the Arctic islands. The migration of marine animals from the north, which had been so conspicuous in the Upper Triassic, was interrupted, and an influx of European forms probably by way of the South American coast, took its place, and the Middle Jura appears to have retained the same connections.

The line between Middle and Upper Jurassic is not always drawn in the same place; by some writers the Callovian is placed in the older and by others in the younger division. In accordance with the arrangement in the table, the Upper Jurassic is here considered as beginning with the Callovian and is marked by extensive geographical changes in the southern and western regions of the continent. Mexico was very largely depressed beneath the sea, except, probably, a belt along the western coast. The small area of marine Jurassic which has quite lately been found in western Texas, is doubtless an outlier of this Mexican transgression. In the northwestern United States was formed an extensive gulf or shallow bay, which covered most of Montana, Wyoming as far east as the Black Hills, and Utah. The beds reach their maximum thickness, 1800 feet, in the Wasatch Mountains, which is reduced to 800 in the Black Hills. The sediments laid down in this bay are limestones, shales, and marls, while the presence of gypsum shows that salt lagoons were formed by isolation of parts of the bay.

The fossils are of Boreal types, like those of Alaska and Siberia, and point to an incursion from the north or northwest, though the oceanic connections of this Jurassic bay have not yet been determined, but the difference between this Boreal fauna and that of California, which is of central European character, makes any direct communication between the two areas unlikely. In the Upper Jurassic'of California, of somewhat later date than the interior bay, the land connection between Alaska and Asia appears to have been again interrupted, opening the way for a current of cooler water to flow from the Arctic down the American coast. At all events, a Boreal fauna, with animals which were then abundant in northern Europe and Siberia, extended its range through California to southern Mexico, though the difference in the fossils shows that some barrier, however low and shifting, prevented the waters of the Atlantic from meeting those of the Pacific. It is remarkable that these northern animals should have extended their range so much farther south on the American than on the Asiatic coast, but the explanation is probably to be found in the cool current from the north mentioned above.

Similar cases are known at the present time, in which the distribution of marine animals is controlled by warmer and colder currents in the sea.

The western interior gulf did not persist throughout the Upper Jurassic, but was drained by an elevation and was succeeded by a continental formation, partly fluviatile, partly, it may be, lacustrine, the Morrison.1 The beds have quite a different distribution from those of the marine Jurassic and extend down the eastern front of the Rocky Mountains from Wyoming to Texas and New Mexico, with disconnected areas, perhaps outliers, in the Black Hills and western Colorado. The Morrison has yielded a remarkable fauna of Dinosaurs and Mammals, which have made it the object of great interest to the palaeontologist. Its exact geological position has been vigorously discussed and it has been referred now to the uppermost Jurassic, again to the lowest Cretaceous. It has been suggested by Professor Williston that different areas of the Morrison are of different dates, just as we saw that the Millstone Grit (Upper Carboniferous) of the Mississippi valley is not a single uniform bed, but different beds of similar character, formed successively and corresponding to several horizons in the great mass of the Appalachian Pottsville. On this view, which is probably the solution of the problem, the Morrison includes several distinct horizons, extending from the Upper Jurassic into the Lower Cretaceous, but the discrimination of these horizons is yet to be made.

1 This is the formation called in the first edition of this book the Como stage and referred to the Cretaceous. The term Como must be abandoned in favour of Morrison, which was first proposed. " The question whether the Morrison formation is Jurassic or Cretaceous is still to be answered, and if a satisfactory answer is ever received it will doubtless be from vertebrate paleontology." (Stanton.) The stratigraphic evidence leaves it open whether these beds should be called Jurassic or Cretaceous and, on the evidence of the mammals, they are here included chiefly in the Jurassic.

In British Columbia, where the Lower and Middle Jurassic are not known, the Upper Jura occurs and shows a transgression of the sea to the eastward of the Cascade Mountains. Here and in California the rocks are prevailingly slates, with included diabase tuffs, demonstrating volcanic activity.

The close of the Jurassic in North America was a time of extensive orogenic movements on the western side of the continent, comparable to the Appalachian revolution which closed the Palaeozoic era on the eastern side, though not of such far-reaching extent and consequences. The Sierra Nevada had long been a slowly subsiding geosynclinal trough, in which great thicknesses of sediment had accumulated. At the end of the Jura it yielded to the forces of lateral compression and rose into a folded mountain chain, transferring the coast-line from the eastern to the western side of the mountains. Farther north, the Klamath and Cascade ranges probably participated in the movement, as did the Coast Range, which seems to have become a chain of islands. Little is known regarding this primary condition of the Sierra Nevada, which may then have been of no great height, and which was not yet separated from the Great Basin by faults, the present mountains being due to long subsequent events. The Humboldt Range in Nevada appears to have had its beginning in the same disturbances, while a widespread movement of elevation affected the interior, though this movement began at a somewhat earlier date.