botany. For example, the collections of the United States National Museum embrace over 100,000 specimens of the impressions of Paleozoic plants, whereas of those showing internal structure there is hardly a half dozen unit trays full. In the Mesozoic and Cenozoic collections belonging to the same institution there are thousands upon thousands of specimens from hundreds of localities and horizons, while of those retaining their internal structure there are so few that they can almost be numbered in tens.

There is another and an excellent practical reason why the impressions of plants are, and will always remain, of more value to geology than those exhibiting internal structure, no matter how well this structure may be preserved. As soon as a plant impression is exhumed it is instantly ready for study and may be interrogated at once as to the stratigraphic story it has to tell, whereas the plant with the structure preserved usually shows little or nothing on a superficial examination, and requires laborious, expensive preparation before it can be identified. For example-to make a personal application-for the past five years I have annually studied and reported on from 500 to 700 collections, each of which embraced from one to hundreds of individuals, and with them have helped the geologists to fix perhaps 50 horizons in a dozen States. If it had been necessary to cut sections of these specimens before the geologist could have had his answer, it is safe to say that very little would have been accomplished.

All fossil plants must be interpreted by and through the living flora. In the more recent geological horizons the plants are naturally found to be most closely related to those now living, but as we proceed backward in time the resemblances grow less and less, and finally we find ourselves in the presence of floras a large percentage of which are without known or clearly recognized living representatives. In describing these and making them available for stratigraphic use it has been necessary to give them generic and specific names, after the analogy of the living floras, so that we may have convenient handles by which to use them. Many of these are confessedly what may be called genera of convenience, such, for example, being many of the genera of the so-called "ferns" of the Paleozoic. Some-but especially botanists-unfamiliar with the geological use of fossil plants have argued that it is unsafe, or even actually unwise, to venture to give names, not only to those without living representatives, but even to those obviously belonging to living groups. A reply to this objection seems unnecessary in view of what has been said.

The practical application of fossil plants as an aid to geology may be briefly mentioned. There have been described from-let us say— North America, upwards of 5,000 species, of which number some

1,200 are confined to the Paleozoic, perhaps 2,000 to the Mesozoic, and 1,500 to the Cenozoic. During the 60 or 70 years that this information has been accumulating it has developed that certain species or other groups enjoy a considerable time range, and therefore are of little value in answering close questions of age, while others are of such limited vertical distribution that their presence may indicate instantly a definite horizon. Thus, if he find in association impressions that we have named Sequoia Nordenskiöldi, Thuya interrupta, Populus cuneata, etc., it is known instantly that we are dealing with the lower Eocene Fort Union formation, since not one of these species, together with several hundred others, has ever been found outside this horizon. Innumerable other concrete examples could of course be given, though hardly necessary, yet it may be instructive to note that within a single geographic province-the Rocky Mountain region-the several plant-bearing formations present are characterized as follows: The Kootenai by 120 species, the Colorado by perhaps 50 species, the Dakota by 460 species, the Montana by 150 species, the Laramie by 140 species, the Arapahoe by 30 species, the Denver by more than 140 species, the Fort Union by from 500 to 700 species, etc. This shows that, as Prof. J. W. Judd once said, "We still regard fossils as the 'medals of creation,' and certain types of life we take to be as truly characteristic of definite periods as the coins which bear the image and superscription of a Roman emperor or of a Saxon king."

Just a word may be said on the economic application of stratigraphic paleontology. It is perhaps safe to say that never in the history of American geology has there been so close an interrelation and dependence of geology on paleontology as at present, and of this confidence paleobotany may justly claim its full share. Thus, of the even dozen of paleontologists in the employ of the United States Geological Survey and covering all branches of the subject, four are paleobotanists.

Among the many subsidiary problems connected with the application of paleobotany to geology, the use of fossil plants as indices of past climate occupies a most important place. As the majority of plants are attached to the substratum and hence are unable to migrate like most animals when the temperature of their habitat becomes unfavorable, they must either give way or adapt themselves gradually to the changed conditions of their environment. Therefore, fossil plants have always been accorded first place as indices of past climates. "They are," as Dr. Asa Gray has said, "the thermometers of the ages, by which climatic extremes and climate in general through long periods are best measured."

To those who have not given especial consideration to the subject, the idea appears to obtain that climatic variations, such as now

exist, are normal or essential, and that they were present without marked differences during all geological ages. It is now established, however, that this conclusion is entirely without geological or paleobotanical warrant, and that the most pronounced climatic differentiation the world has known extends only from the Pliocene to the present. As a matter of fact we of to-day are living in the glacial epoch in what possibly is only an interglacial period, and we know that the time which has elapsed since the close of the last ice invasion has been of less duration than was one, and possibly two, of the Pleistocene interglacial periods. We also know that the climate was milder during these interglacial intervals than has obtained since the final retreat of the ice, as shown by the fact that in eastern North America certain species of plants then reached a point some 150 miles farther north in the Don Valley than they have since been able to attain. The development of strongly marked climatic zones, at least between the polar circles, is, then, "exceptional and abnormal, and we have no evidence that in any other post-Silurian period, with the possible exception of the Permo-Carboniferous period, has the climatic distribution and segregation of life been so highly differentiated and complicated as in post-Tertiary times." 1

The regular and normal conditions which have existed for vastly the greater part of geologic time have been marked by relative uniformity, mildness, and comparative equability of climate. This is abundantly shown by the almost world-wide distribution and remarkable uniformity of the older floras. When, for instance, we find the middle Jurassic flora extending in practical uniformity from King Karls Land, 82° N., to Louis Philippe Land, 63° S., we have conditions which not only bespeak a practically continuous land bridge, but exceptionally uniform climatic conditions. To have made this possible there could have been neither frigid polar regions nor a torrid equatorial belt, such as now exist. The absence of growth rings in the stems of these plants, as well as the presence of such warmth-loving forms as cycads and tree ferns, point to the absence of seasons and the presence of mild and equable climatic conditions.

Another example of similar import is afforded by the early Pennsylvanian flora; that is, the flora of the lower part of the Upper Carboniferous. Wherever terrigerous beds of this age have been discovered, representatives of this peculiar flora, which includes such common genera as Lepidodendron, Sigillaria, Sphenophyllum, etc., have been found, this distribution ranging from South Africa to Brazil and Argentina, and thence over the northern hemisphere.

Similarly, the Mississippian flora (Lower Carboniferous) has been found in Spitzbergen, Greenland, and arctic Alaska, and thence

1 See White and Knowlton, Science, n. s., vol. 31, 1910, p. 760. 85360°-SM 1912

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south over Europe and America, and although somewhat older. than the last, is distinctly related to that in Argentina.

On passing up in the geologic time scale we find that during late Mesozoic and early Cenozoic time the present dominant types of vegetation were firmly established. With what probability of success may these floras be interrogated as to the climatic conditions under which they existed? We find from a study of the present flora that certain types of vegetation, as well as certain plant associations, have definite climatic requirements. Thus, Artocarpus, or the bread fruit trees, are now confined to within 20° of the Tropics, showing that they require the moist heat of the torrid regions. If, now, we find that Artocarpus once throve in Greenland, 70° or more north, during Cretaceous time, we feel justified in assuming that its climatic requirements were not very different from those of its living representatives. And when we find that it was then in association, as it is to-day, with cycads, tree ferns, cinnamons, palms, and other distinctly tropical forms we are confirmed in the opinion that at that time Greenland must have enjoyed a tropical or at least a subtropical climate.

Another example is afforded by the Fort Union formation. In the rocks of this horizon, which now occur on the wind-swept, almost treeless plains of the Dakotas, Wyoming, and Montana and thence northward to the valley of the Mackenzie, are found remains of Sequoia, Taxodium, Thuya, Ulmus, Populus, Vitis, Platanus, Sapindus, Viburnum, Corylus, Juglans, Hicoria, etc. From this array we feel justified in assuming a cool to mild temperate climate for this early Eocene flora, and further, from the presence of numerous, often thick, beds of lignite, that there was a much higher precipitation than at present.

A layer of fan-palm leaves a foot in thickness in a formation in northern Washington indicates climatic requirements in which the minimum temperature did not fall much if any below 42° F. The presence of numerous West Indian types in the Miocene lake beds of Florissant, Colo., would alone point to almost tropical conditions, but as these are associated with others of more northern affinities, it seems safe to predicate at least a warm temperate or possibly subtropical climate.

GEOPHYSICAL RESEARCH.1

By ARTHUR L. DAY.

To write the history of the earth is a very different undertaking from writing the history of a people. In the latter case, a diligent seeker can usually find some ancient monastery where farsighted historians of an earlier generation have collected the more important records which he requires, and placed them within reach of his hand. With the earth's history, which is the province of geology, it is another matter. The great globe has been millions of years in the making, and except for a mere fragment of its most recent history, it has had neither a historian nor an observer. Its formation has not only extended over an almost incomprehensible interval of time, but we have no parallel in our limited experience to help us to understand its complicated development, and no system of classification adequate to the task even of grouping in an orderly way all the observed rock and mineral formations with reference to the forces which molded them. And even if we could correctly interpret all the visible rock records we are still quite helpless to comprehend all those earlier activities of the formation period, whose record is now obliterated.

To the student of the earth's history, therefore, the problem of gathering and ordering such a widely scattered and heterogeneous collection of effects and causes is one of somewhat overwhelming scope and complication. In the industrial world a situation of this kind soon results in replacing individual effort with collective effort in the organization of a system of a scope more appropriate to the magnitude of the task. We are familiar with industrial organization and the wonderful progress in the development of American industries which has everywhere followed it. We are also familiar with organized geological surveys and the success which has attended them in geological and topographical classification. But the idea of organizing research to meet a scientific situation of extraordinary scope and complexity is still comparatively new. The very words "science" and "research" are still regarded as referring to something out of the ordinary, something to be withheld from the common gaze, 1 Presidential address delivered at the 700th meeting of the Philosophical Society of Washington, November 25, 1911. Reprinted by permission from Journal of the Washington Academy of Sciences, vol. 1, No. 9, December 4, 1911, pp. 247-260.