GEOLOGICAL FORMATIONS The geological column has been divided into four great sec tions known as eras or groups and beginning with the oldest are known as the Eozoic, Paleozoic, Mesozoic and Cenozoic Eras. Each era is divided into several ages called periods or systems, which are in turn further subdivided into epochs or series. (Table I.) Eozoic Era The oldest rocks are classified in the Eozoic Era, which has two separate and distinct periods, known as the Archean and Algonkian.
Archean. The Archean rocks, or the basal complex, are highly folded and metamorphosed, containing secondary struc tures, being time and again intruded by still older plutonic rocks.
Algonkian. Algonkian rocks are also extensively folded, metamorphosed and intruded to a large extent, the difference between the Algonkian and Archean rocks is largely due to the possibility of arranging the Algonkian rocks with respect to well-known stratigraphic methods. The presence of sedimentary deposits are known in the Algonkian. No distinct animal or plant remains are found in the Archean and but rather few fossils have been found in the Algonkian, but not enough to warrant any attempt to subdivide the Era on the basis of remains that might indicate the presence of "life" in the system. A great unconformity exists between the two systems, strongly marked by basal conglomerates. The Era as a whole forms the foundation upon which all other rocks rest and no doubt from which they were derived.
The general nature of the Eozoic proves it to be unsuitable for oil and gas, mainly on account of the highly metamorphosed condition of the rocks. The Era is often referred to as the Pre Cambrian.
Paleozoic Era The oldest fossil-bearing sedimentary rocks are in the Paleo zoic Era, or the Era of Ancient Life. The subdivisions of this group from the oldest upward are: the Cambrian, Ordovician, Silurian, Devonian and Carboniferous. In the beginning of this Era life was of the lower order and sea-weeds predominated; towards the close, however, we have a rather high order in the amphibians, or land and water animals. A high order of vegeta tion prevailed at the close of the Paleozoic. Most of the rocks are of marine origin, although fresh-water and swamp deposits are also known. In general, the rocks appear to have been deposited
mostly in shallow water. The beginning and end of the Era is well marked by unconformities caused by great disturbances of the Earth's crust.
Cambrian. The first appearance of trilobites was in the Cambrian, and due to the fact that three different members are characteristic of the system, the zones in which they are found have been correspondingly named, viz.: Olenellus or Georgian Group (Lower Cambrian), Paradoxides or Acadian (Middle Cambrian), and Dikellocephalus or Potsdam Group, (Upper Cambrian). There are also in the Lower Cambrian indistinct remains of sea-weeds, a few sponges, graptolites and corals and some very small pteropods (foot-winged mollusks, or brachiopods). Articulates, tracks and ripple marks are found in the Potsdam, which are indications of shallow water deposition.
The Cambrian so far has not been a producing horizon for oil and gas and it does not seem likely that it will ever prove to be of any importance. However, as only a small portion of the Cambrian is accessible for prospecting, as most formations of this period lie at great depths and the present methods of drilling are inadequate to reach these horizons, the Cambrian cannot be said to have been properly tested.
Ordovician. The whole of this system may be placed in a period before any vertebrate life came on earth, and the system is characterized by the large number of shells and corals that were developed during this time. Bry•ozoa, not found in the Cambrian, are abundant in this period. The Trilobites of this period have a much larger tail shield and can roll themselves up; and have rounder and better developed faceted eyes. Early Ordovician rocks show evidence of being formed in shallow water, with upheavals towards the close. A subsequent formation of lime stones, and the thickening of the formations to the east would indicate that the upheaval was followed by subsidence. The Ordovician is noted for the culmination of the graptolites and they are the most characteristic fossils of this system.