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692 to be appreciated by communities and public bodies. In 1820 a geological survey of the county of Albany, N. Y., was made under the direction of the agricultural society of the county by Prof. Amos Eaton and Dr. T. E. Beck. Two years afterward Rensselaer and Saratoga counties were also thus explored. Prof. Eaton was also engaged by Gen. Stephen Van Rensselaer to make at his expense a geological survey of the country adjacent to the Erie canal. The result of this was published in 1824 in a report of 160 pp. 8vo, with a profile section of the rock formations from the Atlantic ocean through Massachusetts and New York to Lake Erie, the Rev. Edward Hitchcock furnishing many of the details through Massachusetts. The first geological survey made by state authority was that of North Carolina in 1824 and 1825, by Denison Olmsted. Since that time there have been various surveys by the different states or by the federal government, of which we shall notice the most important historically. Beginning at the northeast, early surveys were made of Maine, New Hampshire, and Rhode Island, by Dr. C. T. Jackson, in 1836-'41; of Massachusetts, by Edward Hitchcock, in 1830-'40; of Connecticut, by J. G. Percival and C. U. Shepard, in 1836, and of Vermont in 1845-'6, a work which was continued by Edward Hitchcock and his son, C. H. Hitchcock, in 1858-'60, the latter of whom is now (1874) engaged in a resurvey of New Hampshire. In 1836 was commenced the survey by H. D. Rogers and his assistants of the state of Pennsylvania, which was not completed till 1855. The survey of New York in 1836-'42, by Vanuxem, Emmons, Mather, and Hall, may be said to have opened a new era in American geology by giving a complete and systematic classification of the palæozoic rocks within its borders, which has served as a basis for all subsequent work to the east of the Rocky mountains. The description of the organic remains of the state by Prof. James Hall is still incomplete, but five large quarto volumes have been published. The surveys of Michigan in 1837-'46 by Houghton, and of the Lake Superior region in 1847-'9 by Jackson, and subsequently by J. D. Whitney and J. W. Foster, served to extend our knowledge of the palæozoic rocks to the westward. From that time to the present systematic surveys of the various states of the great Mississippi valley have been or still are in progress, and have already given us a pretty accurate knowledge of the geology of the whole of this vast region. The history of this work is too long for the present occasion, and it may seem invidious to mention names among workers in this great field; but a prominent place should be given, in addition to those just mentioned, to D. D. Owen, B. F. Shumard, Swallow, J. T. Hodge, Worthen, Newberry, Safford, E. W. Hilgard, Cox, and Tuomey. Nor should the important labors of Oscar Lieber in South Carolina and of Emmons in North Carolina be forgotten,

nor the elaborate survey of Virginia by William B. Rogers, of which only partial reports have been published. The geology of the western portion of our continent presents characters widely different from that already noticed, and is now attracting great attention. Much important information was gathered by the labors of W. P. Blake and J. S. Newberry in the course of the great railroad surveys undertaken by the national government; and the geological work has been continued in the important survey of the 40th parallel under Clarence King, and that of the Rocky mountain region by J. V. Hayden. These labors are still in progress, as is also a geological survey of California under J. D. Whitney, and the great geological features of this region are being rapidly made known. Much progress has also been made in the study of the geology of British North America. A geological survey of Canada, embracing the present provinces of Ontario and Quebec, was begun in 1842 under Sir W. E. Logan, with whom were associated for many years Mr. Alexander Murray and Dr. T. Sterry Hunt. In 1870 Mr. A. R. C. Selwyn succeeded Logan in the present Dominion of Canada, including the British territory west to the Pacific, the field of the survey being thus greatly extended. The provinces of Nova Scotia and New Brunswick were early examined by Gesner, since which time Matthew Bailey, Hartt, Hind, Hunt, and Dawson have done much to develop their geology. The last named has especially studied the carboniferous rocks of that region. A survey of Newfoundland is in progress under Alexander Murray. The labors of the late Sir John Richardson, Hector, Hind, and others, have done much to elucidate the structure of the great region north of Canada, until lately known as the Hudson Bay territory.—With this brief sketch of the progress of geological research in North America, we may now proceed to discuss the general principles of geological classification, and to illustrate them by especial reference to American geology. The great groups introduced by Werner remain essentially unchanged, but many alterations in nomenclature and various subdivisions and reclassifications have since been adopted, some of which require notice. Besides the great distinction between crystalline and uncrystalline rocks is that of stratified and unstratified rocks, having reference not to their intimate structure, but to their geognostical relations. The stratified rocks include all those which appear to be arranged in beds or strata, whether crystalline or not; and the unstratified, those which, like granites, traps, basalts, and volcanic lavas, occur in masses which are destitute of such arrangement, and appear to have been forced into their present position while in a more or less softened or molten condition. These are often spoken of as eruptive, irruptive, or intrusive rocks. They are with a few exceptions crystalline, and in certain cases are not readily