CRUST OF THE EARTH—GEOLOGICAL SYSTEMS—EOZOIC, PRIMARY, SECONDARY, TERTIARY, PREHISTORIC FORMATIONS.
Fig. 646.—Systems.
The crust of Great Britain has been carefully examined, and from the results of investigations at various periods, the earth has been divided into a series of strata which follow the same order of succession. Sometimes certain strata may not be present, and they may be replaced by others, but the same order of succession will be found. The order is as follows, commencing at the lowest. The illustration is taken in the opposite direction:—
| “Eozoic.” | ⌈Laurentian ⌊Cambrian | Until recently believed to be without traces of living creatures; hence “Eozoic.” |
| Palæzoic, or Primary. | ⌈Silurian ⌊Cambrian ⎢Carboniferous ⌊Permian | Shell-fish, seaweed, ferns, fish, low reptiles. |
| Mesozoic, or Secondary. | ⌈Triassic (Upper Red Sandstone) ⎢Oolitic ⌊Cretaceous | Birds, marsupials, reptiles. |
| Kainzoic, or Tertiary. | ⌈Eocene ⎢Miocene ⎢Pliocene ⌊Post-tertiary | Superior life. Mammals, with great vegetable life, on to plants and animals now existing. Man. |
| Quaternary. | Recent—Prehistoric |
The Palæzoic Systems.
Laurentian System. It will be perceived from the above list that the Laurentian Rocks are the oldest. The name is derived from the St. Lawrence formations, and was given to the strata by Sir William Logan. They are metamorphosed rocks older than the Cambrian. These rocks are sedimentary, of very old deposition, and of a crystalline nature, consisting of quartz, gneiss, etc. The granite was probably formed by the fusion of its component constituents, quartz, mica, and felspar, which become crystallized by the excessive heat. For a long time no traces of organisms could be detected in this or the Cambrian systems, but modern research has been rewarded with a little success. The original deposits of micaceous gneiss, etc., have been altered, and many true igneous rocks, such as syenite and granite, are found in them. These very old rocks must have been originally deposited in strata converted by heat and pressure into crystalline rocks. These rocks have been divided into two series, under the names of lower and upper Laurentian. They are metamorphic, and consist “mainly of gneiss interstratified with mica-schist, with great beds of quartz, and massive beds of crystalline limestone, of which one varies from 700 to 1,500 feet in thickness. Conglomerates also occur, and there are vast deposits of magnetic and specular iron. Graphite, or blacklead, is disseminated in strings, veins, and beds through hundreds of feet of the lower Laurentian, and its amount is calculated by Dr. Dawson to be equal in quantity to the coal seams of an equal area of the carboniferous rocks” (Nicholson).
Fig. 647.—Upward Granite (Section).
