The Ordovician has not been a great producer of oil and gas, the Trenton Limestone fields of Ohio-Indiana are the only extensive pools known in it. A small amount of gas with a little oil has been found in the St. Peter sandstone, below the Trenton. The Trenton production is found where the limestone takes a dolomitic form.
Silurian. The Silurian period commenced with a submer gence, when large limestone beds were formed, with thickening of the strata towards the east. The subsequent formation of the Salina beds would indicate another upheaval, during which the salt beds were formed in shallow water. A few fish appear in the Silurian, although corals still predominate. Crinoids make their first appearance. The chain coral Ilalysites is characteristic of the period. On the whole, we have a great marine period until towards the close, when marine conditions have been gradually replaced by continental conditions, shales passing into sandstones.
The oil and gas of the Silurian seems to be restricted to the Medina formation; the "Clinton" sand fields of Ohio and Ontario are of this age, and are believed to be part of the Medina sandstone rather than a continuation of the Clinton limestone. The gas so far seems to have exceeded the amount of oil that has been produced, although valuable discoveries of more oil are being constantly made. On the whole the Silurian has not been an extensive oil- or gas-producing age.
Devonian. The rocks of this period show that there was a considerable raising and lowering of the surface so that at one time the land was above water and at another time below. It was a great volcanic period and during it the earth's crust was disturbed by one of the greatest epochs of mountain formation. The seas were full of invertebrate forms similar to the Silurian's, although the Cystoids were almost extinct. Trilobites were fewer and had a characteristic ornamentation of the margin of the head as well as extraordinary development of spines which many display on the hcad and tail shield. Corals and brachiopods were in large numbers and new form of cephalopods appeared. Vertebrates, such as fishes, were well developed, and hence the Devonian is sometimes called the Age of Fishes.
The Devonian is one of the greatest gas-producing periods, and the upper half of it has considerable oil production in the Appalachian fields.
Carboniferous. The last period of the Paleozoic is divided into three subdivisions, which make up the entire Carboniferous; they are the Mississippian, Pennsylvanian and Permian.
The Mississippian system as a whole shows signs that the vegetation of the Carboniferous has been on a high plane. The presence of coal beds are first noted and they had been formed from the immense vegetation. Amphibians or lung and gill animals, that could live in water or on land make their first appearance, and they gave rise to reptiles towards the close of the period. Shales and sandstones form a great part of the Mississippian, but at the close of the epoch the formation of an extensive limestone is noted, generally referred to as the sub carboniferous limestone, although different parts of it, as they make their appearance in various places in the country, are grouped and known under different names. During this period trilobites are still found, but become entirely extinct at the end of the Paleozoic. Phillipsia is a rare but characteristic trilobite of this age. A wide spread upheaval is noted at the close of the Mississippian. Cystoids disappear entirely in the Carboniferous, while Blastoids are abundant and- characteristic of the Carbon iferous limestones; Crinoids culminate their development while the Bryozoa Archimedes is characteristic of the Lower Carbo niferous.
The gas production of the Mississippian is almost equal to the production in the Devonian, and the oil production has doubled that of the Devonian.
The Pennsylvanian is known as the Coal Age, as the best coal deposits are known to have been formed during this period. A generalized section of the Pennsylvanian shows that it is composed mainly of alternate layers of shales, sandstones, limestones and coal deposits. A great amount of oil and gas has been produced from rocks of this period, and the combined volume of the Mississippian and Pennsylvanian is exceeded only by the Tertiary deposits of foreign countries.