To Louis Agassiz is due the honor of launching upon the scientific world the general theory of glaciation. To be sure, others, a little while before the appearance of his great work on "Etudes sur les Glaciers," had attained something of the main idea in limited areas; but it was the work of the great Swiss widely to apply the principles and develop the conception into a grand general hypothesis. It took twenty years to get the theory firmly established. The last two decades have been devoted to accumulating facts and solving problems connected with the causes and effects produced by the various phases of the great ice movements. The glacialists have been, during this time, the most active and numerous of any class of geologists. During the last ten years no branch of geology has produced so voluminous a literature. The subject is constantly expanding at an astounding rate. All earth students are brought more or less closely into contact with glacial phenomena; many geologists are devoting all their energies in this direction. In Europe the names most prominently associated with glacial work are those of Penck, Geikie, Croll, and Schmidt-in this country those of Chamberlain, McGee, Dawson, and Leverett. As long as geology lasts the works of these scientists will remain classics.

Twenty-five years ago the main geological features of the upper Mississippi basin were deciphered with great difficulty on account of the heavy deposits of "“drift” covering the whole country. Strangely enough, this very region has become, glacially, the most interesting in the whole world. Glacial history was here first found to be long, complex, and full of stirring incidents. Instead of a single ice period, no less than half a dozen great glacial drift sheets are now known to exist. To this region the eyes of the world are now directed for a complete time-scale of ice movements, with which glacial deposits in all parts of the globe may be compared. Stranger still is the discovery of evidence of other great ice ages in remote geological

times. Regarding these, we yet know little of what the future has in store for us.

William Smith's discoveries concerning the fossils in the rocks were epoch-making in directions other than purely geological. Fifty years of active interest accumulated a vast mass of facts. Comparisons of successive faunas showed that the later-formed rocks contained organic remains very nearly like those living, but that as we examine older and older strata the fossils become more and more unlike present forms.

About the middle of the century embryological studies were making great progress. It was at this time that the ancient organisms began to have an unusual interest to the biologist. It was widely thought that in the fossil types could be secured forms that would represent all the early stages of the living organisms. Erroneous as this hypothesis proved to be, it long served to stimulate, to an extraordinary degree, the study of the fossils from the biotic standpoint. It eventually removed paleontology from the domain of geology and united it with biology.

When Darwin's "Origin of Species" appeared, in 1859, a new light was thrown upon the vast body of largely unconnected facts that had been long accumulating regarding the fossils. These facts furnished some of the strongest proofs of the theory of descent. With the new way of looking upon the organic remains of past ages there arose an active search for specific pedigrees. The results were startling. One has only to mention, among many, the work of Marsh on the horse, of Cope on the camels, of Naumayr on mollusks, or of Gaudry on the cats.

In studying the evolution of existing organisms, natural selection is the great factor to be considered. The fossils emphasize a wholly different set of laws. It is this fact, chiefly, that divides the evolutionists into two great schools, known as the Darwinists and the Lamarckians. The one gives greater prominence to natural selection, the other to physical environment.

While the testimony of the fossils has been a great boon to the biologist, it has, as perfecting a detailed and universal scheme of geological chronology, proved rather disappointing. Careful surveys of various zoological classes of fossil organisms now show that there is little or nothing intrinsically to locate a given form in time. This must be derived from the known superposition of the rocks. This phase of the subject has been dwelt upon lately by our distinguished countrymen, Wachmuth and Springer, who have gone into the most exhaustive investigation of a large group-the crinoids-that has ever been made.

Another singular fact to be mentioned in this connection concerns the time that organisms have been on earth. It has generally been conceded that at the beginning of the fossil record, in the early Cambrian, organisms as a whole were already at least nine-tenths differentiated. Consequently, they were regarded as existing eons before. Brooks has recently pointed out that, while organisms change very slowly in the open sea, along the shore their differentiation goes on with great rapidity owing to the intensity of the struggle for existence. Hence, the very marked divergence of animal types at the time when our earliest known records were made militates strongly against long previous careers. On the other hand, this radical separation of the main types may have taken place during a comparatively short period.

Whatever contributes greatly to the amelioration of man's condition must always have an honorable place among grand achievements. To have one of the greatest industries brought under rational control is a feat comparable to any discovery in pure science, or any production of art. A new epoch begins with the placing of mining upon a basis truly scientific. From time immemorial, mining has been the great game of chance. Vastly more wealth has been put into the earth annually than has been taken out through this channel. Up to the time of the establishment of modern geology there was an excuse for haphazard min

ing. Although the ordinary miner is still slow to grasp the advantages held out to him by science, every large mining enterprise now has the geology of the district carefully examined before the first shovelful of dirt is turned. Most of the great mining companies even have a regular geological corps employed all the time. One of the greatest petroleum concerns at present operating owes its immense success, not so much to questionable business methods, as so often claimed, as to far-sightedness in employing at a large salary the scientist who worked out, by purely scientific methods, the geology of natural oil.

The esteem with which pure geology is held by intelligent people is further shown by the support of special governmental bureaus, called geological surveys, by nearly every civilized State. An officer of one of the largest iron companies of the Lake Superior region recently said that if he and his associates had only placed even a little faith in the State geological survey twenty years ago, they would have saved hundreds of thousands of dollars. This company, as myriads of others do constantly, then believed firmly in "practical" work, not in "scientific" work-as if science, rightly applied, were not preeminently practical, and as if it were not, as Huxley says, only enlightened com

mon sense.

Mining to-day is capable of being put on as secure a business foundation as any manufacturing enterprise. From start to finish it may be carried on with the certainty and despatch of the running of a railroad train or an ocean liner. Lucky "finds" will, however, continue to be made; but henceforward mining as a business will no longer be a vast lottery-ever developing, to their fullest extent, the gambling propensities of mankind.

The microscope revealed a new world to the student of animals and plants. Its later use in the study of rocks brought to light another world, equally undreamed of and equally vast. Modern geology began in a consideration of the materials with which the subject has to deal. For the

first three-fourths of the nineteenth century little progress was made over what was bequeathed by the century preceding. This branch of the science had come practically to a standstill. Other branches sprang up and grew so rapidly that inorganic geology bade fair soon to be relegated to a very subordinate place.

When, then, it was discovered that when thin plates of rocks were magnified under polarized light their minutest mineral constituents could be identified, the most powerful of weapons was placed in the hands of the geologist. At a single bound inorganic geology took its place by the side of the organic branch. A century ago the study of rocks had gone as far as it could because laboratory methods of examination were crude. But the simple grinding down of rock fragments into thin plates at once overcame a seemingly insurmountable difficulty. Large rock sections are now made with ease, so that equal sizes of the thinnest tissue-paper seem in comparison like thick slices of bread. The blackest lava or basalt in thin sections becomes as transparent as window glass. At a glance the mineral constituents can be told from one another as easily as horses from cattle or sheep in a meadow. Under the microscope the dull gray granites break up into brilliant hues that rival the rainbow. The gorgeous stained-glass windows of many cathedrals give but a faint idea of the wondrous beauty of the rock mosaic. But this is not all. A moment's viewing under the microscope tells the essential chemical and mineralogical composition of a rock with greater accuracy than the most refined chemical analysis. The mineralogical changes that a rock may have gone through are also evident. A rock may be metamorphosed beyond all recognition through ordinary means, but in thin plates its original condition is at once disclosed.

The last twenty-five years of petrography have been devoted largely to the development of working methods. With these labors will always be associated the names of Rosenbusch, Zirkel, Michel-Lévy, and Judd, and in this