In the extremely arid region of Arizona, on the other hand, which is considerably further south, the evidence of the Gila conglomerates indicates that while frost was very active, the increase of precipitation, though undoubtedly present, was comparatively slight. This shows that the climatic balance was not greatly disturbed, the chief effect being an important lowering of temperature, probably due to cold northerly winds. The Gila conglomerates are double, separated by a period representing present-day conditions. Summing up the evidences of glacial climate in North America, we find a striking similarity to Europe. In the north elevation and increased land area caused the development of large ice-sheets, which appeared first in the mountainous regions with a heavy snowfall, and later spread over the drier plains and plateaux of the interior. This first glaciation was long and complex. Owing to the anticyclonic conditions which formed over the ice, the rain- and snow-bearing depressions were forced to pass further southward, causing greater snowfall on the mountains and high water-level in the lake basins. This greater snowfall, together with the cold conditions due to the existence of the ice-sheets to the north, caused the development of mountain glaciers south of the main glaciated region. In the east there were cold northerly winds which carried a severe climate as far south as Florida. This Glacial period was followed by subsidence, and a long spell of dry, moderately warm climate lasting perhaps 200,000 years, after which elevation and glacial conditions again set in. These conditions were not so severe as the first, and their duration was much less, while they were broken up by several intervals of temporary recession of the ice, one of which, corresponding to the Riss-Wurm period, lasted for 60,000 years, and perhaps should be considered as an “interglacial.” This period was marked in its early stages by the deposition of the curious æolian deposit known as “loess,” indicating steppe conditions. After the last glaciation there set in a stage of irregular retreat. BIBLIOGRAPHY Leverett, F. “Comparison of North American and European glacial deposits.” _Zs. f. Gletscherkünde_ 4, 1910, pp. 280, 323. Wright, W. B. “The Quaternary Ice Age.” London, 1914, Chs. 8-9. Attwood, W. W. “The glaciation of the Uinta Mountains.” _J. Geol._, 15, 1907, p. 790. Henderson, J. “Extinct glaciers of Colorado.” _Colo. Univ. Studies_, 3, 1905, p. 39. Gilbert, G. K. “Lake Bonneville.” Washington, _U.S. Geol. Survey Monograph I_, 1890. Russell, I. C. “The geological history of Lake Lahontan.” Washington, _U.S. Geol. Survey Monograph XI_, 1885. Coleman, A. P. “An estimate of post-Glacial and interglacial time in North America.” _Rep. 12 Internat. Congr. Geol._, 1913, p. 435. CHAPTER IX CENTRAL AND SOUTH AMERICA