STRATIGRAPHICAL GEOLOGY. 3 group of rocks which, in the British Islands or the Celtic province, have no existence or “ homotaxial ” representative, such a group necessarily would be best described from its known locality. The greater part of the oldest and most widely-distributed of the Archaean rocks occurs on the North-American Continent. For the complete elucidation of their history, we still await the researches of American geologists ; and if their past labour and progress afford or indicate a measure of future efforts, grand indeed will be the result. Divisions of Geological Time.—In describing the great groups of stratified rocks or formations, it must be understood that their names are commonly used to designate the successive periods of time in vhich they were formed. For convenience, all geological time has been divided (and naturally so) into three great groups, or the succession of periods known as Primary, Secondary, and Tertiary. Equivalent but more classic terms, based upon life distribution, are Palaeozoic, Mesozoic, and Cainozoic, signifying respectively ancient life, middle or less ancient life, and recent or modern life. These names were proposed by Pro fessor John Phillips, and are now generally adopted. They essentially refer to the presence and distribution of organic life through time. Primary Period.—The Primary period refers simply to that which preceded the Secondary, being the most ancient or first great cycle of time of which we at present know anything, but by no means the first or beginning of all time ; of that we know nothing. There is no definite starting-point in the so-called Primary or Palaozoic period; our chronological commencement is lost in the remote past. Secondary Period.—The Secondary period naturally succeeds the Primary. In this great division of time, however, we have a recog nised base and close, a beginning and ending; both boundary lines being definitely governed by life, as well as by recognised stratigraphical or physical changes, producing unconformity and discordant strikes. Tertiary Period.—The Tertiary period, in like manner, has its special characteristics ; no species that had occurred in the Secondary era, lived on or passed into the Tertiary sea. In other words, no Secondary form passed through these changes which mark, and ceased with, the close of the Cretaceous period, and commencement of the Tertiary epoch. Geological time, as before stated, is thus arranged : We may either commence our researches downwards through these three great life divisions, or we may begin from the unknown base—the Azoic or Archaean time of Dana, the pre-Cambrian, Cambrian, and Silurian eras of other physicists—and read upwards through the recognised successive changes that have taken place during the long history of these Palaeozoic, Mesozoic, and Cainozoic epochs. Neozoic Time.—It was suggested by the late Professor Edward Forbes, both upon palaeontological, physical, and petrological grounds, that it might be advantageous to obliterate the division between the Secondary and Tertiary periods, thus dividing geological time into two great epochs instead of three, viz., Palaeozoic and Neozoic (Old