the impressions then received were in mind when, in after years, I had my attention drawn to the several theories advanced for explaining the causes which produced the warm and frigid epochs. But, so far as my marine experience goes, such theories have not harmonized with nature’s mode of operating at this age of the world. Therefore, I have conceived views which, to my mind, are more agreeable to the simple operations of nature of which I have long been witness. Consequently, I have written several short essays on climatic changes since 1880, and also letters relating to the same subject, which have been published in _Science_ and _Scientific American_. But the space allowed for the introduction of such matter was necessarily too limited for so wide an explanation as the subject required. The views then advanced I have again repeated, with the addition of several facts pertaining to physical geography, which, so far as I know, have never before been published.” WAKEFIELD, MASS., U.S.A. June, 1896. CONTENTS. CHAPTER I. PAGE CAUSE OF COLD AND MILD PERIODS, 9–36 Traces of ancient glaciers in temperate zones, 9; prevailing winds the main cause of the circulation of the ocean waters between the tropical and temperate zones, 10; general direction of prevailing winds, and how, in connection with continents, they circulate the surface waters of the sea, 11; high and low sea-levels; separation of antarctic lands from South America, 12; Captain Larsen’s discoveries in antarctic regions, 13; how low lands south of Cape Horn were submerged, 13; how the winds move more surface water southward than northward, 14; Dr. Croll’s views on winds and ocean currents, 16; under-currents of the ocean, and how caused, 16; Gulf Stream currents, 17; antarctic under-currents, 18; why the winds were able to force more of the ocean waters southward than northward at the close of the Tertiary age, 19; Mr. Alfred R. Wallace’s views on Tertiary seas, 20; how the Cape Horn channel affects the ocean currents, 21; cause of the increase of cold in southern latitudes, 22; how the Cape Horn channel is closed during ice age, and its effect on ocean currents and temperature of southern latitudes, 24; the melting of glaciers from southern lands, 27; a salt sea requisite for circulation during ice age, 28; direction of surface currents in southern seas, 29; Humboldt current, 30; Agulhas current, 32; temperature of arctic ice, 34; movement of southern icebergs, 35; glaciers south of Cape Horn, 36. CHAPTER II. HOW ICE PERIODS IN THE NORTHERN HEMISPHERE ARE BROUGHT ABOUT, 37–54