The Permian is not well developed in this country, and it really marks the transitory period between the Paleozoic and Mesozoic times. The division point between these two great ages is marked by great continental changes. The Appalachian uplift of the Eastern part of the country and the upheaval of the Utah Basin region as well as the submergence of the Nevada Basin were the characteristics of the Permian.
In this country the Permian has been found nearly barren of oil and gas, although a small amount has been found in the lower part of it. Most of the Permian is composed of depositions formed under semi-arid conditions (continental deposits), and as such are generally wanting in organic remains, they are not favorable for oil and gas.
Mesozoic Era The Mesozoic Era may be described briefly as the Age of Reptiles. Two subdivisions are made, namely the Jura-Trias and the Cretaceous. This Era is noteworthy for the extensive eruptions at the close of the period.
Jura-Trias. A striking contrast is had between the beginning of the Jura-Trias and the close of the Paleozoic; the latter with its numerous upheavals seems to have settled down to a long period of gentle geographical change in the Jurassic. The Triassic has yielded the first known mammals, although they were not very flourishing in the Mesozoic. Archeopteryx, the first known bird, made its appearance in the Jura-Trias. The close of the Jura-Trias was marked by numerous upheavals.
The oil and gas possibilities of this epoch, as well as the lower half of the next one (Cretaceous), are similar to the Permian, and so far as the United States are concerned, very be expected of them.
Cretaceous. The formation of many chalk deposits gave the name to this period; in it some of the largest reptiles have been found and many well-preserved specimens have been obtained. The deposits of the Cretaceous arc somewhat limited in the eastern part of the country, but are well developed in the west; in it we find many fresh-water limestones and lignite deposits.
The upper part of the Cretaceous has produced gas and oil, the main areas being Caddo, La., Colorado, Wyoming, Dakota and the Calgary, Alberta fields.
Cenozoic Era The most recent subdivision of the geological column is known as the Cenozoic Era, and deals with "Recent Life," or the age of Mammals and Man. The Cenozoic is divided into two great periods, the Tertiary and Quaternary. They are further subdivided into Eocene, Miocene and Pliocene for the Tertiary, and Pleistocene (Glacial) and Holocene periods for the Quaternary. • A large amount of oil in the United States is obtained from the Tertiary and the total production is exceeded slightly only by the combined production of the Mississippian and Pennsylva nian. With the exception of the Mexican Cretaceous oil, all the foreign production is from the Tertiary. Tertiary oils are found mainly in unconsolidated sands, which yield enormous quantities of oil owing to the unconsolidated nature of the sand grains, which therefore have a large percentage of effective porosity.