tions over continental glaciers will be obtained through assembling the observations recorded in the journals of sledging parties, notably the wind direction, the orientation of sastrugi, temperature conditions, snow surface and humidity.

Dr. Simpson's discussion of the Antarctic blizzard suffers especially because in his evaluation of the available moisture content of the air over the Antarctic region he has taken account, not of the water content in all states of aggregation within the unit of space, but of that portion only which is uncongealed and therefore registered by the usual hygrometric apparatus. Adiabatic changes readily transform the ice needles of the cirri into moisture, which until again congealed or crystallized is duly registered upon the hygrometric record, and if the congealed material is not to be entered in the accounting the proper equating of available moisture before and after an adiabatic meteorological change will be impossible. This is obviously the reason why Dr. Simpson says of my theory of the glacial anticyclone that it fails "to explain the origin of the precipitation and the mechanism of blizzards," and "Hobbs has left unsolved . . . the greatest problem of the Antarctic anticyclone, namely, the origin of the precipitation within the anticyclone." As a matter of fact, in all my papers upon the subject this process of precipitation and its origin in the blizzard itself has been elaborated, not as an additional feature of the anticyclone, but as a necessary inherent quality which can in no wise be omitted.

The continental glacier in its evolution must be conceived to have developed from enlargement of an ice-cap which is nourished like the mountain glaciers by ascending moist air currents. Ice-cap glaciers exist today on Iceland, in Norway, and as isolated masses on the borders of continental glaciers. Though in form they resemble continental glaciers, they are: (1) much smaller, (2) they are nourished by a wholly different process, (3) they owe their existence to their location upon a pedestal-like base which is relatively flat and extends above the snow line of the region.

An interesting question arises, "At what stage of their development will this type of glacier take on the auto-circulation of a continental glacier and by reaching up draw down the ice needles of the

cirri for its nourishment?" When will it first make its own weather instead of taking that which is brought to it by travelling whirls? A partial answer seems to be afforded by two glaciers, the Vatnajökull of Iceland and the inland-ice of Northeast Land, Spitzbergen. Some years ago Dr. Thoroddsen, who of all scientists was best acquainted with the Vatnajökull, informed me that it possessed no auto-circulation but appeared to be entirely dominated by the vigorous cyclonic whirls which summer and winter are located over the sea to the southwestward. This mass of ice is roughly elliptical in shape with major and minor axes of about eighty and fifty miles respectively and has an area of 8,500 square km. Subsequent observations by other observers lead me, however, to hold this as perhaps doubtful. A larger ice mass which is more nearly circular and with corresponding axes of one hundred and eighty miles is the inland-ice over Northeast Land, Spitzbergen. The narrative of Baron Nordenskiöld, who crossed it in 1873, clearly reveals the evidence of anticyclonic conditions.

The great glacial anticyclones of Greenland and the Antarctic can hardly be considered apart from their important rôle as parts of the earth's planetary system of winds-they perform an important service in turning backward toward the tropics those high level currents which have passed the horse latitudes and have not yet been brought down to the surface, in doing which they remove the moisture which had been locked up in the ice needles of the cirri. They undoubtedly lend vigor to the entire circulatory system and thus accentuate the climatic zones. When in the Pleistocene age far greater continental glaciers lay over North America and Northern. Europe, the climatic zones must have been still more strongly marked. There appears to be in this an explanation of the fact, now generally recognized, that in the geologic past climatic barriers have prevented the poleward or equatorward migrations of sensitive organisms only during such geologic periods as were characterized by extensive glaciation and presumably by correspondingly strong Evolution of Geologic Climate," Bull. Geol. Soc. Am., Vol. 30, 1919, pp. 499–566.

See F. H. Knowlton,

66

climatic zonation, as I pointed out in 1916. We are living today in the waning hemicycle of one of these abnormal glacial periods, and as a consequence have reconstructed much of the past geologic history, not upon the standard or typical pattern of the earth, but upon a markedly abnormal, short-lived and seldom occurring set of conditions which happens to be the one in which we are living.

UNIVERSITY OF MICHIGAN,

September 14, 1920.

7 W. H. Hobbs, "The Ferrel Doctrine of Polar Calms and its Disproof in Recent Observations," Proc. Second Pan-Am. Sci. Congress, Washington, 1915-16, sec. 2, pp. 185–187.