The Forest Flora in the Mountains. The influence of the glaciers on the distribution of trees in the Rocky Mountains and the Sierra Nevada Mountains of California was most marked. Let us trace the history of the vegetation of this region before considering this point. We have already alluded to the extension of the sea which in the upper Cretaceous separated the western part of North America from the eastern. The Great Basin during the Lias was an enclosed natural lake, which was gradually filled by the action of aerial agents. Further during the early Cretaceous, the region of Missouri was covered by a sea and in Nebraska, there are evidences of a long continuance of a freshwater lake. It is also probable, that the prairie region in the Miocene period was covered water. With the drying up of these seas and inland lakes, the prairie region was formed and the plant associations following the retreat of the water culminated in a prairie grass formation, which still further acted as an influence in the separation of the eastern and western floras. The factors mentioned above were sufficient to introduce a differentiation into the floras of the two widely separated regions and the separation of the tree vegetation of North America into eastern and western types may be said to have begun. That this separation was not fully accomplished until, after the Glacial period is proved by the presence of remains of Sequoia trees in the eastern United States of a later date. The climate of the eastern slopes of the Rocky Mountains during the upper Cretaceous period and subsequent must have been more humid than the climate of today, which is comparatively arid. We have proof of this climate in the petrified forests of Arizona, where the trunks of gigantic trees have been preserved unlike the flora of that region at the present day. In the soft shale rock at Florissant, Colorado are found fossil leaves, fruits and twigs of trees clearly allied to the living redwoods or sequoias of California, to oaks, hornbeams, alders, walnuts, chestnuts, elms, ashes, sumachs, hollies and other trees and shrubs arguing for a different climate in the far past. The oncoming of the glacial period and consequent refrigeration produced even a more marked change in the distribution of tree vegetation than the inland sea of upper Cretaceous times. With the development of the continental glacier, as the ice sheets spread from the two great centers of accumulation, they united in the region north of lakes Superior and Huron. With their near approach to the lakes the area of conifers was divided into an eastern and western section. The trees of the western section were submitted to the action of the local mountain glaciers and their areas of distribution were thus broken up and many of the species of eastern affinity were destroyed and others were restricted within narrow limits. This has not been worked out for all the species of western trees, but the principles may be illustrated by a discussion of the glacial and post glacial distribution. Of the giant trees of California Sequoia gigantea (= S. Washingtoniana) JOHN MUIR') observed that the location.

1) MUIR, JOHN: On the post-glacial History of Sequoia gigantea. Proceedings American Association for Advancement Science 1876: 242-253.

of the Sequoia forests was on the general forest soil-belt between the individual Sierra mountain glaciers. The remarkable gap between the northern and southern groves is located exactly in the pathway of the vast mer de glace of the San Joaquin and King's River basins. The other great gap in the belt, forty miles wide, extending between the Calaveras and Tuolumne groves, occurs exactly in the pathway of the great mer de glace of the Tuolumne and Stanislaus basins, and that the smaller gap between the Merced and Mariposa groves occur in the pathway of the Merced glacier. We are, therefore, forced to conclude that the Sequoia trees which were widespread in Miocene times were restricted by the action of the glacial ice to the California side of the continent and that the remnant of a once continuous forest was still further separated into isolated groves by the action of local glaciers, such as filled the basins of the San Joaquin, Tuolumne and Merced rivers. The action of the glaciers on the distribution of other forest trees, illustrated by this clear cut example, must have been similar and we have as a result the separation of the original forest of North America into the Atlantic and Pacific types. A comparison of the coniferous vegetation of the Rocky Mountains, of eastern North America and of the Pacific coast is to be found afterwards (Chapter IV). A similar difference shows itself in a comparison of the broad-leaved vegetation of eastern and western North America. When during the glacial period, the greater part of North America was covered with ice and later submerged in its central plain region beneath the sea, it is not strange that the peculiarities of the western and eastern floras manifested, as far back, as Cretaceous times should be further accentuated. Many characteristic genera of the Atlantic slopes of North America are missing in California, and the prairie region, viz., Asimina, Zanthoxylon, Stuartia, Gordonia, Tilia, Robinia, Gleditschia, Gymnocladus, Cladrastis, Nyssa, Liquidambar, Viburnum, Clethra, Ilex, Catalpa, Diospyros, Sassafras, Benzoin, Carya (Hicoria), Morus, Ulmus, Fagus, Castanea, Carpinus, Betula, Magnolia, Liriodendron. There are lacking also many trees which from the Tertiary period to the present maintained themselves in North America. Common and related species of the following genera occur in eastern and western North America: Aesculus, Acer, Rhamnus, Ceanothus, Ptelea, Euonymus, Acer, Negundo, Staphylea, Rhus, Sophora, Cercis, Prunus, Pirus, Crataegus, Amelanchier, Calycanthus, Philadelphus, Ribes, Cornus, Sambucus, Viburnum, Symphoricarpos, Lonicera, Cephalanthus, Gaultheria, Kalmia, Styrax, Fraxinus, Platanus. On the other hand, the plants mentioned below are peculiar to western America: Fremontia californica, Larrea mexicana, Cneoridium dumosum, Zizyphus Parryi, Karwinskia Humboldtiana, Adolphia californica, Glossopetalon nevadense, Prosopis, Parkinsonia with two species, Canotia holacantha, Charpentiera californica, Whipplea modesta, Menodora with many species. That this separation took place during and subsequent to the glacial period is proved by the discovery in the Pliocene deposits of California of remains of Platanus occidentalis and species of Magnolia. LESQUEREUX') has discovered at Golden 1) LESQUEREUX in Hayden Report 1872: 371-427.

Calaveras Big Tree Grove, Sierra Nevada, California. are known as "Professor Gray" and "Dr. Torrey." "Grizzly Giant" is in the background.

City Colorado two fossil species of Platanus, three of Juglans, one of Ulmus, one of Lindera (Benzoin), one of Sapindus and one of Fagus. None of these genera exist in Colorado at the present day.

Prairies. With the gradual disappearance of the sea which extended over the central prairie region during the Miocene period, the soil at first was largely impregnated with the salts of sea water, which were afterward leached out by the rain which with the drying up of the land fell with less constancy, producing more arid conditions on the eastern slopes of the Rocky Mountains. HARVEY') suggests that on account of the low precipitation this region bounded on the west by the crest of the Rocky Mountains was denied at first to tree invasion and came to be occupied by a prairie formation, which was displaced after the glacial period by the invasion of trees, while the plains became grasscovered. The formation of the prairies introduced a drier climate in the interior of North America and new elements of plant life were introduced which developed into the numerous and peculiar forms, characteristic of the prairie region of today. Numerous Cactaceae, many Chenopodiaceae, the Chlorideae, the peculiar Polemoniaceae of the prairies appeared during Miocene and Pliocene times.

Coast Plains. We have thus far traced the developments of the main types of the North American flora during the Glacial period. Before treating of the post glacial and present distribution of North American plants, let us briefly allude to the conditions which probably existed on the Atlantic and Gulf coast plains.

During the Tertiary period that portion of North America which now comprises the Atlantic and Gulf coast plains was beneath the Atlantic Ocean. Later during the upper Tertiary, it was elevated excepting the portion comprising the Florida peninsula. With its appearance above the sca, it was tenanted from two main sources of plant supply, viz., the flora which covered the elevated mountain and table lands of the present eastern states as far north as New York and the coastal flora which must have been differentiated and fringed the tertiary sea coast composed of typic sea coast plants. The sandy plains of the coast were tenanted chiefly by pines which if they did not arise as coastal plain species probably were derived from the near, by tertiary forest so frequently referred to. The assortment of species in the newly formed land depended largely upon edaphic conditions. The sandy soil of the coastal plain would only support those species which had previously existed in the near by land areas under similar edaphic conditions, which may have been a rocky ledge of some mountain side, or the sandy bottom of some silt filled. ravine, or which were sufficiently plastic, as species, to adapt themselves to the new surroundings.

Colony of northern plants in Florida. The topography of Florida in the neighborhood of the Appalachicola River is in striking contrast to the level

1) Botanical Gazette XLVI: 84.