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as they could, with the corresponding groups of other countries. Such is the method ever since pursued, by which our knowledge of the strata which make up the outer crust of the earth has been systematically extended. The importance of the organic remains found in the rocks has been more and more appreciated, and the shells constituting the chief portion of these have been most thoroughly studied; for while the different formations or groups of strata may contain numerous similar beds of limestone, sandstone, slates, and shales, not to be distinguished by their mineral characters, and which frequently cannot be traced to their meeting with other known formations by which their place or relative positions may be determined, the fossils show no such indiscriminate distribution. Each period was characterized by its peculiar group of animated beings, and if their arrangement is understood it follows that the position of any stratum in which the fossils are recognized must also be determined. A single species may in some cases be peculiar to one member of a geological formation, and serve wherever the fossil is found to identify the rock; but usually in different countries their identification by fossils is dependent upon characteristic genera and the order of succession of their principal groups. This branch of the subject will be more particularly treated in the article Palæontology.—In the latter part of the last and early part of the present century papers upon geological subjects occasionally appeared in the transactions of the American philosophical society of Philadelphia, the transactions of the American academy, and in other scientific journals. The character of these papers is almost exclusively descriptive. There is, however, a theory of the earth proposed by Franklin in the “Philosophical Transactions” of 1793; and in vol. vi. appeared the memorable essay of William Maclure, read Jan. 20, 1809, entitled “Observations on the Geology of the United States, explanatory of a Geological Map.” The author of this paper had undertaken a more arduous and gigantic work even than that which was occupying William Smith of England; it was no less than a geological survey of the United States alone and at his sole expense—a work which entitled him to the appellation he has received of the father of American geology. In this pursuit he crossed the Alleghanies fifty times, visited almost every state and territory in the Union, and for years continued his labors mostly among those who could have no appreciation of his objects. He had visited nearly all the mining districts of Europe, and thus was well qualified, for one of that period, to recognize the corresponding formations of the two continents. He traced out the great groups of strata then designated as the transition, secondary, and alluvial, in their range from the St. Lawrence to the gulf of Mexico. The tertiary, however, he did not recognize, owing to the absence of the chalk formation, the upper member of the secondary, which in Europe, being largely developed and most conspicuous, marks the strata of more recent origin lying above it as tertiary. He continued his explorations after this report, and in May, 1817, presented another to the philosophical society, accompanied by a colored map and sections. His observations were also extended in 1816 and 1817 to the Antilles, and a paper upon the geology of these islands was published in the first volume of the “Journal of the Academy of Natural Sciences.” Prof. Silliman of New Haven, educated to the profession of the law, was induced by President Dwight of Yale college to qualify himself for the departments of natural science, particularly chemistry; and with this view he spent some time previous to 1806 in England and Scotland. In Edinburgh he became familiar with the discussions of the Wernerians and Huttonians in that transition period, as he styles it, between the epoch of geological hypothesis and dreams and the era of strict philosophical induction in which the geologists of the present day are trained. The interest excited by this controversy could not fail to direct his tastes toward the new science, and he returned to become its zealous promoter, for half a century or more aiding to elucidate the geology of his country, inspiring the enthusiasm of others, and furnishing in the “American Journal of Science” an organ for the diffusion of scientific knowledge. At that period (1804-'5), he says, geology was less known in the United States than mineralogy. Most of the rocks were without a name, except so far as they were quarried for economical purposes, and classification of the strata was quite unknown. Dr. Archibald Bruce of New York commenced in 1810 the publication of a journal devoted principally to mineralogy and geology, the earliest purely scientific journal supported by original American communications. It was well received at home and abroad, but appeared only at wide intervals, and ended with the fourth number. The mineralogical collections at the principal colleges, and others belonging to scientific men mostly in New York, promoted inquiry and observation concerning the geological relations of the minerals and their distribution. The admirable treatise on mineralogy by Prof. Parker Cleaveland, published in 1816, fostered while it gratified this spirit of inquiry. In 1818 the brothers Prof. J. F. Dana and Dr. Samuel L. Dana published a detailed report on the mineralogy and geology of the vicinity of Boston. In the same year was first published the “American Journal of Science,” which has continued ever since to be the chief periodical American recorder of the progress of the sciences. The next year the American geological society held its first meeting at New Haven, where it continued to meet annually for several years. The importance of geological explorations, with the view of thereby ascertaining the agricultural and mineral capacities of large districts, was be-