Martha's Vineyard and Barnstable (Mass.). The circle of 36° embraces Yucatan and Honduras; and, reaching nearly to Lake Nicaragua, it encloses several islands near our Pacific coast, besides taking in part of Alaska and Greenland. The northwest peninsula of North America and the northeast peninsula of Asia appear to belong rather to northern Occanica.

4. Africa: Regarding this continent, suffice it to remark that a radius of 24° extends from the Tropic of Cancer to the Tropic of Capricorn, passing between Lakes Albert and Victoria, Nyanza, over Mt. Mfumhiro (10,000 ft. high) and Mt. Omatako (8,800 ft. high). A radius of 36° not only defines the continent, passing through the Straits of Gibraltar, Mediterranean and Red Seas, Mozambique Channel, aiding to separate Madagascar from Africa, or to elevate it at a later period; thence round the Cape of Good Hope, enclosing the islands of St. Helena, Ascension, St. Paul, Capes Verde and Canary, but also passes between Madeira and Porto Santo (where Lyell found a continental difference in the mollusks, see Principles, II Vol., p. 427) and reaches the place of beginning. Much of the belt passes through Cenozoic regions and excludes the Quaternary of Madagascar, with its gigantic Epyornis, just as a similar radius in Australasia, excludes the Quaternary of New Zealand, with its Dinornis, etc.

It would occupy too much space to go into details regarding South America, Australasia and Oceanica; but as far as examined, the law applies there also.

IV. Fourth subdivision of the law: Besides these three modifying influences, toward the close of the Mesozoic and beginning of the Cenozoic, the western Alps became a dynamic focus, reaching, according to Elie de Beaumont, their present height during the Miocene Period. Mount Rosa is nearly, if not quite, the geographical centre of the entire dry land on the globe; and the Alps connect with the Himalayas and Andes, of similar geological age, by a great circle or belt of immense seismic activity, viz., along the Carpathians and flanks of the Caucasus, near the disturbed regions of the Caspian, through Persia to the southern slopes of the Himalayas; thence through the volcanic islands of the Bay of Bengal, the numerous volcanoes of Sumatra and Java, near Sumbawa's earthquake region to that of New Zealand. This belt or zone, on the opposite side of the globe, reaches that portion of the Andes, where almost daily rumblings are heard (about Chimborazo); thence, crossing the earthquake region of Vene

zuela, marks the Bitumen Lake of Trinidad, and many active volcanoes of other West India Islands, as well as of the Azores.

Demonstration: A radius of 9° from Mt. Rosa defines accurately the Miocene Tertiary (as given on Murchison's map) on the east coast of England, also in the middle of Denmark; through Prussia it is Eocene; but Miocene again through Austria, S. Italy, Sicily, Algiers and vertically central Spain. With a radius of 12o, we may follow the force which probably aided in the separation of Ireland from England, and which is indicated by other important breaks or depressions. With a radius of 36° we pass, from the Miocene on the eastern flanks of the Urals, to that of Spitzbergen and of Greenland. Contracting this radius somewhat, we follow the Carboniferous and Permian of Russia also to Spitzbergen and Greenland. The great circle, pointed out in the law, as passing from the Alps to the Himalayas and Andes, marks chiefly tertiary regions.

Summary: The dynamics of land-forming would seem, from the foregoing demonstrations, to comprise first, a longitudinal force, scarcely, if at all, seismic, adding to continents chiefly by aqueous depositions, inasmuch as each northern continent, near the termination of the median line, has a large river delta. Secondly, there is an arctic-antarctic force mostly along continental coast lines, and connected with active seismic phenomena of alternate elevations and depressions: apparently from these being thinner or more yielding portions of the earth's crust than at continental elongations. Thirdly, in each continent there are radii and circles, connected with one or two important foci, which have aided in defining the geographical limits of each continent, and in bringing geological deposits, in successive curves of increase to, or near the surface: possibly because the wave impulse, directly under the upheaved plastic focus, sends its molten contents to equidistant circles, beneath the plastic crust.

The geology of each continent, but especially Europe, has also been somewhat modified by the Alpine central focus of the dry land hemisphere.

As corollaries, attention may be called to two additional great circles of activity, which are secondaries and great circles of illumination to those phases of the ecliptic which would present themselves, if we arrest the globe, respectively at the summer and winter solstice, when the meridian of Mount Rosa (the centre of

land) marks noon on June 21 and December 21. This gives us one great circle from Behring's Straits to its antipodal antarctic, due south of Mount Rosa; almost defining the junction between the land and water hemispheres; the other from Scandinavia, at the Arctic circle, to the antipodal point, on the Antarctic, which will be found due south from Behring's Straits. As these run through the northern hemisphere, the course of the more active one is from the volcanoes of Sumatra, nearly parallel to the Asiatic continental trend, formerly described, thus skirting the Japan Gulf Stream; thence they pass again, nearly parallel to the west coast trend, through North and South America; whereby a region or zone is marked of Nevada geysers, Arkansas and Virginia Hot Springs, Yucatan, Cuban, Venezuelan, Grenadan, Peruvian and Chilian volcanic and earthquake regions. The course of the other, while running nearly parallel to the North American east coast trend, is from near the thirty-nine volcanoes of Central America to the geysers and volcanoes of Iceland, thus skirting, and probably aiding to heat, our Gulf Stream, while on the opposite side of the globe, this zone or belt passes near the Hindoo Kush and western Himalaya elevations, the disturbed regions of Hindostan and islands in the Bay of Bengal (some brought up within the historical period) as well as the numerous volcanoes of Sumatra.

As corollary of the law that, as one continent comes up, there is usually a depression on the opposite side of the globe, let us follow the eastern Himalayas, north through the high Asiatic Tablelands and observe on the opposite side the depressions of Hudson's Bay, our fresh-water lakes, the Gulf of Mexico and the deep southeast Pacific, devoid of islands. May not this great depth of ocean, unobstructed by continents on the west, enable the moon to create here the initial, tidal wavė, by raising the water in this part of the ocean, somewhat above the general level, chiefly to follow her apparent path west, but also to send a smaller wave, obstructed partly by the coast of Patagonia, to extend round Cape Horn and there meet the main west coming tide?

The evident connection of these laws with Terrestrial Magnetism, with Mineralogy and economic Geology, Archæology and Ethnology, I leave for others to elaborate; or possibly, if this paper is considered as furnishing sufficient demonstrations to point to a law, further elucidations bearing on these collateral branches may be attempted on my part, should health be spared.

ON MAXIMUM SYNCHRONOUS GLACIATION. By W. J. McGEE, of Farley, Iowa.

INTRODUCTION AND SYNOPSIS.

THE present paper may be considered as a review of what is known as the "ice-cap theory." It is true that this theory, as originally propounded by Adhemar, has been virtually abandoned; but modifications of essentially the same theory are yet advocated by some of the ablest geologists. Some analysis of the underlying principles may not, therefore, be deemed inappropriate.

In Part I the agencies which effect glaciation are briefly outlined; and it is shown from the mode of operation of these agencies that precipitation over the central portions of any extensive ice-field must be very slight or even nil. Applying the same principles to the polar zones of the earth, it is shown that under no conceivable circumstances could ice accumulate very thickly around the poles. A glance at the present condition and past history (so far as known) of the polar regions, then indicates that there is no sufficient reason for believing that the polar regions were ever much more extensively glaciated than at present.

Part II is designed principally to show approximately (1) the relative temperature of all parts of the earth when the ice of a glacial epoch extends to any latitude; (2) the greatest possible accumulation of ice at any latitude; and (3) that the presence of an ice-field at any parallel is inimical to precipitation at any higher latitude. For the sake of simplicity it is, in three of the sections of this Part, assumed that the climatal and meteorological conditions are identical on all parts of the same parallel ; or, what is equivalent, that the isotherms and isobars are parallel and separated by constantly uniform distances. Such a condition would be much more favorable to extensive glaciation than that which actually obtains. The influence which would be exerted on the earth's centre of gravity by such an ice-cap as that of which the dimensions are calculated has accordingly not been computed, as it is undoubtedly many times too large. Lest the correctness of the figures representing temperatures under varying conditions should be questioned, an additional section (Sec. III), showing

that the results are in perfect harmony with known meteorological phenomena, is appended.

In Part III the nature and cause of ice-motion are discussed, with the object of showing that the assumption of a vast polar ice-cap to force outward the peripheral portion of the ice is unnecessary and incompetent. The viscous theory of ice-motion is adopted, with some modifications; and some of the most serious objections to the theory are considered. As this part is to an extent speculative, it may be stated that the general conclusions reached are independent of it, and will stand even if the viscous theory of ice-motion be shown to be incorrect.

PART I.

SEC. I. SUMMARY OF PRINCIPLES.

1. CONDITIONS OF THE PROBLEM.

Heat is the first essential.—The researches of recent years have shown that a diminution of the temperature of the earth, as a whole, could not produce glaciation. As shown by Tyndall, every pound of ice piled up on land represents an amount of heat sufficient to melt five pounds of cast-iron; and the true difficulty lies in accounting for the heat stored up in the ice of the glacial epoch. But heat alone will not effect glaciation.

Necessity for a condenser." It is quite manifest that the thing most needed to produce the glaciers is an improved condenser; we cannot afford to lose an iota of solar action; we need, if anything, more vapor, but we need a condenser so powerful, that this vapor, instead of falling in liquid showers to the earth, shall be so far reduced in temperature as to descend in snow. The problem, I think, is thus narrowed to the precise issue on which its solution depends."2

Evaporation and precipitation do not occur together.—Certain meteorological conditions accompany profuse evaporation, while precisely opposite conditions result from rapid condensation. In the first case the air is necessarily humid; and as the presence of aqueous vapor in the atmosphere checks radiation from the earth and from the lower atmospheric strata, the earth's surface and the Heat as a Mode of Motion,” Am. ed., p. 176; "Forms of Water," p. 153. 2 Heat" etc., p. 176.