Knowing that the present physical agencies at work on the globe have been acting through long ages,[6] it can readily be appreciated how small effects have been accumulated and low elevations, for instance, have become immense, high mountain ranges. The growing mountain ranges alter the climate and the meteorological conditions. The rainfall on one side differs from that on the other. The temperature varies with the altitude, and so on. Although continents have gradually and steadily grown from the earliest times, there have been many local alterations of land and sea. Marginal sea bottoms have become great mountain ranges. Islands have appeared and sunk from view. Lakes have been gradually converted into solid land or into peat bogs. Fresh water bodies have become brackish. Dry lands have become marshes, and forests have been buried beneath the waves. Geologic changes have caused great alterations in climate at given times and in given areas. These statements may be illustrated and emphasized by a brief reference to the =development of the Continent of North America=. This Continent has grown from comparatively a small beginning to its present great proportions. In doing so it has passed through eras of stupendous duration. These eras in the order of their occurrence are as follows: (1) =Archæan era=; (2) =Palæozoic era= (subdivided into =Cambrian=, =Silurian=, =Devonian= and =Carboniferous= periods); (3) =Mesozoic era= (subdivided into =Triassic=, =Jurassic= and =Cretaceous= periods); (4) =Cenozoic era= (subdivided into =Tertiary= and =Quaternary= periods); the Tertiary is subdivided into =Eocene=, =Miocene= and =Pliocene= epochs; the Quaternary is subdivided into =Glacial=, =Champlain= and =Terrace= epochs; (5) =Psychozoic era=, or recent epoch.