mineralogy and palæontology, the ores and mines, and in general the natural economics, resources and physical geography of the country. The topographic branch prepares the maps; the irrigation branch investigates the possibilities of irrigation and selects the irrigable lands and sites available for reservoirs and canals. The work of the topographic branch is the basis of the work of the other two, and all the results of the latter are projected on the maps. The publications of the survey are: (1) the annual report of the director, which, besides the administrative report, contains memoirs on geologic subjects by members of the survey, and is distributed according to the regulations of the Interior Department; (2) monographs on the leading subjects of special investigation by the geologists; (3) bulletins on more limited special subjects of research; (4) an annual volume of mineral statistics. The last three are distributed gratuitously only to designated libraries and to learned corporate societies, which send their own publications in exchange. Otherwise they are sold by the director and the money deposited in the treasury. See GEOLOGY; IRRIGATION; TOPOGRAPHY.

Methods and Publications.-The preceding paragraphs state in a general way the functions of this Survey as originally defined by Congress, together with subsequent modifications which included within the scope of its duties the study of the hydrographical conditions relating to water power, and to the irrigation of the arid lands of the western States.

As the geologic and hydrographic work depends upon the topograhpic work, the preparation of a suitable topographic map received primary consideration, and the general lines of operations extended to secure it were very definitely outlined from the earliest period of the Survey, so that at the present time almost the total area of the United States, exclusive of Alaska, has been surveyed and mapped for this purpose.

In the execution of the field work the procedure has conformed to the general methods employed in accurate trigonometrical surveys; but the enormous extent of the territory surveyed; the great diversity and the peculiar arrangement of the natural features of the country, and the necessity for executing the work as expeditiously as possible, and yet consistent with all the requirements of thorough accuracy, have tended to develop methods which are not only specially applicable to the work of the Survey, but also form a group of comparatively new methods available for any other line of topographical work. These methods may be briefly outlined as follows:

The surveying and mapping operations conform to the general plan which divides the whole area of the country into a series of quadrangles each of which is equal to a square degree, that is, each quadrangle is bounded on the east and the west by a degree of longitude, and on the north and the south by a degree of latitude.

The surveying operations consist in the extension of a system of primary and secondary triangles with tertiary triangulation points over the whole country, accompanied by three systems of level lines, supplemented by a system of road and stadia traverse.

The primary triangulation Las been planned

for the control of the work over the whole country, thus insuring the accurate ultimate meeting of fragmentary surveys which may be initiated a hundred or even a thousand miles apart. In this work, the triangles are expanded from accurately measured base lines, and connect various points of reference the geographical positions of which have been accurately determined by the most approved astronomical methods. The astronomical work consists of (1) the measurement of the zenith distances of stars by means of delicate zenith telescopes, for the determination of latitude; (2) the exchange of telegraphic time signals between unknown astronomical positions and a known astronomical position, such as a firstclass observatory, for the determination of the differences of longitude; and (3) the observation of circumpolar stars for the determination of the azimuth of a line, such as a base line, or the side of a primary triangle. The base lines are measured by means of base bars, iced bars, or steel tapes, proper allowances being made for sag, pull, etc., and the measurements repeated several times in order to reduce the probable error to a minimum of less than one in 1,000,000. The elevations of the various stations are established by lines of precise levels run from the datum of mean sea level determined by means of accurate tide gauges. The angles of the triangles connecting these stations are measured by means of theodolites equipped with high power terrestrial telescopes. From the data thus obtained, the lengths of the sides of the various triangles are computed and the entire system of triangulation plotted on the topographic map to furnish the primary control for the secondary detail.

The secondary triangulation is usually exe-· cuted by means of the plane table, and new points located so as to give from one to three good tertiary triangulation points per square mile. The elevations of these points, usually hill summits, are determined by the measurement of vertical angles of elevation and depression, depending upon spirit levelling, while the lower relief of the country is determined by lines of secondary spirit levels run six miles apart with intermediate lines of flying levels run three miles apart with sufficient accuracy to allow them to close on the secondary levels within the limits of one or two feet.

Traverse Lines.- Where the country is covered with dense forests, or where the surface relief is insufficient for triangulation purposes, both the primary and secondary control consists of a system of primary traverses checked by primary triangulation locations, or by astronomically determined positions. These traverses are run by compass and plane table, and a secondary system of traverse lines consisting of odometer and stadia measurements of roads is interwoven with the plane table work. The data obtained from the secondary traverses is plotted upon the plane table sheets during the progress of the work, and is subsequently adjusted upon the final map between the check points established by the primary traverse, or by the plane-table triangulation, the distances between which are so short one to four miles - that errors of location are scarcely perceptible upon map scales of one inch to one or two miles.

The work of primary triangulation and pre

cise levelling is usually executed a season in advance of the topographic sketching, while the secondary triangulation, traverse work and lines of flying levels are immediately followed by the topographic sketching, both classes of work being done by members of the same party. The data obtained from the secondary triangulation and traverses is then plotted upon a sketch sheet. This data is of such a character that each sheet includes from two to five trigonometric locations; from four to eight inches of road traverse; and one or more instrumental elevations, per square inch. Equipped with a sketch sheet thus prepared, the topographer places himself at a point of known elevation and sketches on the sheet by eye with the aid of a hand level the plan of the contour line which passes through his position. In open country, the contours may be located in this manner quite accurately for a distance of half a mile in either direction from his position, corresponding to a total distance of an inch upon the sketch sheet. In wooded country where the figure of the contour cannot be seen beyond his immediate position, he proceeds by road carefully observing the variations of the slope, and determines the differences of elevation for short distances between check points by means of the aneroid, and sketches in the plan of the contour according to the data thus obtained. In cases where the number of accurately determined elevations are insufficient, more locations are fixed by vertical angulation, or by flying levels as the work of sketching progresses. In this manner a system of contours is built up along the roads and watercourses, and if the lines do not practically fill up the entire sheet, the topographer walks into the spaces within the road circuits, and by means of stadia lines for long distances, or by pacing for short distances, determines the positions of the contours required to complete the sheet. The sketches thus obtained are inked in, either in the field or at the office, and are then reduced to the scale of the final map by photography and form the copy for the engravers.

The great topographic map of the United States now being thus prepared by the Survey, is published in atlas sheets of aproximately uniform size, 161⁄2 by 20 inches, on which the mapped area occupies a space 171⁄2 inches in height, and 111⁄2 to 16 inches in width according to the latitude. The division of land represented by an atlas sheet is called a "quadrangle," and is always bounded by parallels of altitude and meridians. Although the sizes of the sheets are always the same, three different scales are employed in the mapping of the surveyed areas in order to serve different purposes and to suit various conditions. A scale of 1: 62500, very nearly one inch to one mile, is used for mapping the thickly settled, or industrially important sections of the country. The sheets on this scale cover an area of 15' of latitude by 15' of longitude. A scale of 1: 125000, very nearly one inch to two miles, is used for mapping the greater part of the country. The sheets on this scale cover an area of 30′ of latitude by 30' of longitude. A scale of 1:250000, very nearly one inch to four miles, is used for mapping the desert regions of the western States, and gives sheets which include an area of 1° of latitude by 1° of longitude.

This map is printed in three colors and

shows the following named classes of natural and artificial features: (1) Water, including seas, lakes, ponds, rivers, creeks, canals, swamps, etc., are shown in blue. (2) Relief, including mountains, hills, valleys, cliffs, etc., are shown in brown contours. (3) Culture, or the works of man, such as villages, towns, cities, roads, railroads, boundary lines, etc., are shown in black. The features shown in blue and black are selfexplanatory. In the case of the brown contour lines, each contour passes through points which have the same altitude above mean sea level, and a series of such lines arranged one above the other at regular vertical intervals, but appearing on the map at irregular intervals, that is, close together where the slopes are steep, and far apart where the slopes are gentle, accurately delineates the general configuration the country, and gives the elevations of all points above the level of the sea. The vertical interval adopted varies according to the character of the country mapped. In a flat country it may be as small as 10 feet, while in a mountainous region it may be as large as 200 feet. usually, every fifth contour is made heavier than the others, and is accompanied by figures giving its elevation above the level of the sea. The heights of many other points, such as the intersections of ordinary roads and highways, railroad crossings and stations, the summits of uplands, hills and mountains, and definite bench marks, are also given in figures which are placed close to the points to which they refer, and are correct to the nearest foot. Each sheet is designated by the name of a principal town, or the name of some prominent natural feature within the district represented, and the names of the adjoining published sheets are printed on the margins. Explanations of the various conventional signs used are printed on the back of each sheet, and materially assist in the reading of the map.

This topographic map is the base on which the facts relating to the geology and the mineral resources of a quadrangle are represented, and constitute the sheets of the Geologic Atlas of the United States published by the Survey. The price of the topographic sheets is five cents each when the number purchased is less than 100 copies, and two cents each when they are ordered in lots of 100 or more copies. In the Geologic Atlas the topographic and the geologic sheets of a quadrangle are bound together, and, accompanied with a textual description of the district represented, constitute a folio of the Atlas. These folios are sold for 25 cents each, except those that have received special treatment and are unusually comprehensive. The price of such varies according to the character of the information afforded, the number of the maps in the folio, etc. All communications relative to these maps, or any other publication of the Survey should be addressed to The Director, United States Geological Survey, Washington, D. C.

The accompanying map is a small portion of the Housatonic quadrangle which includes portions of Massachusetts, New York and Connecticut. It affords a general idea of the topographical treatment; but, being printed only in black and white, it fails almost completely to give a true idea of the actual beauty of the original sheet as printed in the three conventional colors already described.

early recognized as important by reason of their cataclysmic nature; but later and more detailed studies show that quite as important, perhaps vastly more so, are the unobtrusive activities of rain, wind, running water, glaciers, waves, tides, and even organic agencies.

In following out these processes it must be always borne in mind that they operate with exceeding slowness. Indeed it is only through the conception of a history enormously long, expressed in millions of years, that we can understand the truly stupendous character of the work_accomplished. (See section on Age of the Earth). With this vast length of time emphasized in the thought it must be further borne in mind that the great changes of the geologic past have been produced for the most part by the operation of those forces now going on before our very eyes. For that reason, we study closely the changes now taking place in order that in their light we may properly interpret the records read in the rock pages of the geologic volume.

In its broadest sense geology is one of the most inclusive sciences. The life records of the past can be read only in terms of modern biology; the remoter history of the earth's be ginnings are inextricably interwoven with astronomy; physics and mechanics must be invoked to explain tides, interior rigidity, earth heat, and many other problems; the ultimate analysis of the materials of which our globe is composed must be referred to the chemist, as must also many of the changes involved in weathering and metamorphism to be detailed later; meteorology and climatology furnish the only rational background for the adequate study of those external forces now modifying the earth's surface. And so in nearly every respect it is seen that geology overlaps other fundamental sciences.

GEOLOGY SUBDIVIDED

Geology covers an extensive territory and is usually subdivided into the following more or less generally recognized branches.

Cosmic Geology treats of the origin of the earth, its relations to the other bodies of our solar system, and its general relations in space, thus encroaching on the field of astronomy. See COSMOGONY.

Geognosy treats of the materials of which the earth is composed, air, water, and solid crust, known as the atmosphere, the hydrosphere and the lithosphere.

Mineralogy includes a study of the chemical composition, crystal form, origin, and occurrence of the large number of definite chemical compounds (minerals) of which the earth is made up. Optical mineralogy deals with the study of the optical properties of minerals with the polarizing microscope.

Petrology treats of the origin, occurrence, constituent minerals, and texture of the rocks of the earth.

Petrography, used loosely as a synonym for petrology, is in reality a more restricted term applying to a study of the structure, texture, and composition of rocks either macroscopically or microscopically, but not concerned with origin or occurrence. In later usage there is

even

microscopic study.

a tendency to restrict petrography to Lithology, formerly used in the same sense

as petrology, is beginning to be confined by many to the macroscopic study of rocks as contrasted with petrography. See MINERALOGY; METAMORPHIC ROCKS; PETROLOGY; ROCKS; SEDIMENTARY ROCKS; ETC.

Dynamical Geology treats of the forces which tend to change or modify earth structure or earth form. It details the agencies at work and the processes by which they operate. These forces are hypogene (internal) and epigene (surficial). Three great processes are usually recognized, gradation, diastrophism and volcanism. Gradation is surficial and due directly to the action of rain, wind, running water, glaciers, wave work, tides, plants and animals. Volcanism is internal and deals with great movements of fluid rock, the most striking exhibition of which is to be seen in volcanic phenomena. Diastrophism, also internal, treats of the movement and distortion of rock masses (deformation) which the earth's crust undergoes, and which are best known through the manifestations of earthquakes.

Its

Structural Geology, also known as geotectonics, deals with the arrangement of the materials of which the earth is made up. province is to investigate the origin of these structures and their practical importance in applied geology. It has to do with the causes of layering or stratification in rocks, the origin of folds and dislocations, and other problems of similar nature.

Physiography (geomorphology), now generally recognized as a science distinct from geology, deals with the origin and development of land forms, traces out the topographic expression of structure, and embodies a logical history of oceanic basins, and continental elevations; of mountains, plateaus and plains; of hills and valleys. Physical geography is used loosely as a synonym, but the term is more properly applied to the borderland between geography and physiography; dealing, as it does, largely with the human element as influenced by its physiographic surroundings. See GEOMORPHOLOGY; PHYSIOGRAPHY.

Paleontology treats of the life of the geologic past. It outlines the methods by which the evidences of that life are preserved in the rocks, traces in detail the development of various life forms, and attempts to correlate extinct with living genera. It overlaps the fields of botany and zoology, throws enormous light on the problems of evolution, and constitutes the real basis of all efforts to determine the relative ages and relations of strata in widely separated regions (Stratigraphic Correlation). Paleobotany is a sub-branch which deals exclusively with the palæontology of the plant world. See FOSSILS; PALEONTOLOGY; PALEOBOTANY; PETRIFACTION; etc.

Stratigraphy is not infrequently called historical geology though the latter term also properly includes paleontology. It is concerned chiefly in the working out of the history of past geologic ages. One of its problems, as has just been stated, is the correlation of strata in widely separated regions. Without this phase of geology little systematic advance in the science would have been possible.

Economic Geology. Soil, water supply, mineral fuels and oils, building stone, base and precious metals; all these and many other constituents of the earth have been widely ex

ploited by man for his use. Economic geology deals with the application of geologic principles to this exploitation. It is concerned with the distribution, mode of occurrence, mineralogic content, and origin of these economically valuable substances. See COAL; ECONOMIC GEOLOGY; GOLD; SILVER; IRON.

Mining Geology. There has grown up a treatment of applied geology particularly adapted to the mining engineer and known as mining geology. As one of its fundamental phases this obviously embraces economic geology. It also is concerned largely with structural geology, since the miner is interested likewise in the structure of the rocks in which the economically valuable minerals occur.

Glaciology. Glaciers have been responsible for much modification of topographic form over large areas, and as detailed study of existing glaciers and glaciation and glacial phenomena in the past has increased, there has grown up a separate treatment of the subject under the name of glaciology. See GLACIER; GLACIAL PERIOD; PLEISTOCENE EPOCH.

Oceanography. The publication of a vast mass of information resulting from several important deep-sea dredging expeditions has resulted in the development of this branch of the science to such proportions that many writers give it the rank of a separate branch of geology. See OCEANS; DEEP SEA EXPLORATION; CHALLENGER EXPEDITION; etc.

Metamorphic Geology.- Under the influence of favorable conditions in nature many minerals break down and their elements recombine to form new compounds. This process is very extensive and frequently results in the formation of entirely new rock types. The process by which one rock is altered into another is known as metamorphism, and the branch of geology which treats of the cause and nature of this change is known as metamorphic geology. See ROCK CLEeavage; MetamORPHISM; METAMORPHIC ROCKS; Rocks.

DEVELOPMENT OF GEOLOGY AS A SCIENCE.

Probably the earliest geologic phenomena to cause comment were earthquakes, active volcanoes and floods. These were the cause of fruitful, if unscientific, conjecture among the earliest peoples who were wont to ascribe them to the vagaries of mythological monsters. Aristotle believed earthquakes were due to subterranean winds, and recognized a relation between them and volcanoes. Strabo followed in the same belief and going further was able to show the true nature of long dormant or extinct volcanoes. As early as the 6th century B.C. the presence of marine shells far inland was pointed to as an evidence that the land had been elevated from beneath the sea. Somewhat later it was affirmed that rivers eroded valleys and that land emerged from the ocean and was again resubmerged with exceeding slow

ness.

But these advanced ideas took little hold upon the mind of the time and during the Middle Ages the dominance of the Church with its insistent adherence to the exact letter of the biblical story of creation was a strongly deterrent factor in the growth of all scientific thought. The presence of fossils in the rocks was ascribed by some to Noah's flood. Others seriously taught that the Creator made many

unsuccessful attempts before the right forms were finally produced, and that fossils were these rejected forms. Others even insisted that fossils were made by the devil to perplex the faithful.

Nicholas Steno (1631-1687) was one of the earliest observers to work out consistent geological theories. He developed the idea of the marine nature of fossils and showed evidences that the stratified rocks in which they occur are similar to marine sediments now accumulating. He advanced the idea that these rocks, where not now horizontal, must have undergone upheaval and deformation, and cited such folding as one of the chief causes of mountains. Steno had followers, but at best his ideas gained ground slowly.

Many wildly fanciful speculations have been advanced to account for the origin of the earth, but the first serious scientific attempt was made by Descartes about 1644. He conceived the earth to have been at one time a molten mass like the sun, which cooled down to the condition of a molten center with a solid crust. Thus he laid down the foundation of the now famous nebular hypothesis.

Guettard (1715-1786) was the first to make geologic maps showing the distribution of various rock formations and mineral deposits. He also did much to systematize paleontology. He published a work on the erosional effects of running water, citing many specific instances of its efficacy; suggested that the salinity of the sea was due to salts carried to it from the erosion of the land; and was the first to recognize the true nature of the group of extinct volcanoes in Auvergne.

In 1763 Desmarest, a Frenchman, made a careful study of the basalts of Auvergne, and by noting their intimate association with volcanic scoriæ became convinced of their volcanic origin. In this belief he was strongly opposed by Werner (1749-1817), professor of mineralogy and mining at the Freiberg Mining Academy, who was probably the first to attempt to work out a stratigraphic classification of the formations of the earth's crust, which he believed could be applied everywhere. It is essentially as follows: There existed a universal ocean from which the oldest or Primitive Rocks. were chemically precipitated. As the earliest formations in the regions studied by him were granites, gneisses, basalts and other crystalline rocks, he assumed that these were of chemical origin. As the land gradually emerged from the universal ocean, erosion progressed and the Transition Rocks were mixtures of mechanical sediments with the Primitive series. As the waters became still further restricted mechanical deposits predominated to which he gave the name Floetz Rocks. These in turn were followed by the Alluvial series or recent clays, sands and gravels. He maintained strongly the chemical origin of basalts, as did his followers, and around him grew up a school known as the Neptunists, who vigorously argued his beliefs. The adherents of the igneous origin of basalt were also organized into a school under the name of the Plutonists and the controversy between them was for many years a bitter one. Though Werner was an enthusiast, his rigid adherence to his own doctrines in the face of evidence to the contrary probably did much to retard real geologic advance.