Page:The American Cyclopædia (1879) Volume VII.djvu/706

694 admitted the existence of an underlying or basal system of stratified crystalline rocks, which were supposed to be anterior in their formation to the appearance of life upon the earth, and from the apparent absence of fossils were called azoic rocks (signifying without life). In accordance with this nomenclature, the formations containing the fossil remains of plants and animals have been divided into palæozoic, mesozoic, and cenozoic rocks (signifying ancient, middle, and recent life); while subsequent discoveries, indicating that life had already made its appearance in the so-called azoic period, have led to the substitution of the name eozoic (signifying the dawn of life). These four great divisions are made the basis of the accompanying tabular view of geological formations. The subordinate divisions of Cambrian, Silurian, Devonian, &c., are of local origin, which, as will be seen, is also true of the names of most of the formations into which these in their turn are divided. In regard to the palæozoic rocks, which have been most minutely studied in Great Britain and America, the names of the subdivisions recognized in these countries are given side by side. For the details of the mesozoic and cenozoic rocks, which have been made the subject of not less careful analysis and subdivision in Europe, the reader is referred elsewhere. A complete table of them is given on page 109 of Lyell's “Student's Elements of Geology” (1871).

It should, however, be borne in mind that all such divisions of the rocks are arbitrary and

artificial. From the mode in which sediments have been deposited, and from the alternations