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MAPS.
A map showing the water courses and theunevenness of the surface is a topographicmap; and on such a map certain works ofman are usually represented also. Othermaps may be based on topographic maps toshow the relations of other classes of facts totopography.
A map showing the distribution of rockmasses is a geologic map.
The Geological Survey is making a largetopographic map and a large geologic mapof the United States . These large maps arebeing made in small sections or sheets ofconvenient and uniform size. Several thou-sand such sheets are required for the wholeof the United States . Taken together theywill constitute an atlas, and each leaf is calledan atlas sheet.
SCALE.
The area of the United States (withoutAlaska ) is 3,025,000 square miles; a map ofthe United States 240 feet long and 180 feethigh would contain 3,025,000 square inches.Each square mile of ground surface would berepresented by a corresponding square inchof map surface, and one linear mile on theground would be represented by a correspond-ing linear inch on the map. This relationbetween distance in nature and correspondingdistances on the map is called the scale of themap. It is expressed by the phrase “onemile to an inch.” A map of the United States half as long and half as high would have ascale half as great; its scale would be “twomiles to an inch.” Scale is also often ex-pressed as a fraction, of which the numeratoris a length on the map and the denominatoris the corresponding length in nature ex-pressed in the same unit. Thus there are63,360 inches in a mile, and we may expressthe scale “one mile to one inch” by
Three different scales are used on the atlassheets of the U. S. Geological Survey ; thesmallest is the second, and thelargest ^Voo- These correspond approximatelyto four miles, two miles, and one mile ofnatural length to one inch of map length.At the bottom of each atlas sheet the scaleis expressed as a fraction, and it is furtherindicated by a “bar scale,” a line dividedinto parts representing miles and parts ofmiles.
On the scale ^5 one square inch of mapsurface represents and corresponds nearly toone square mile; on the scale of to about
four square miles; and on the scale ofto about sixteen square miles.
ATLAS SHEETS.
A map of the United States on the small-est scale used by the U. S. Geological Survey would be 60 feet long and 45 feet high. Ifdrawn on one of the larger scales it would beeither two times or four times as long andhigh. To make it possible to use such a mapit is divided into parts printed on atlas sheetsof convenient size, about 17 by 21 inches, andbounded by parallels and meridians. Eachsheet on the scale of ^7000 contains one squaredegree (that is, represents an area one degreein extent in each direction); each sheet onthe scale of j^oo contains one-quarter of asquare degree; each sheet on the scale ofone-sixteenth of a square degree. Theseareas correspond nearly to 4,000, 1,000, and250 square miles.
The atlas sheets, being parts of one greatmap, are laid out without reference to politi-cal boundary lines of any kind. They are notstate, county or town maps, but only parts ofone map of the United States . For conven-ience of reference they are given such namesas will readily suggest the region shown.
THE TOPOGRAPHIC MAP.
The features represented in this map aremountains, hills, valleys, meadows, plains,prairies; lakes, ponds, swamps, streams,canals; roads, railroads, cities, villages, bound-aries. They fall into three groups: inequali-ties of surface, or relief; distribution of water,or drainage; the works of man, or culture.
Figures 1 and 2.—Contours in perspective and in horizontal plan or map.
Relief is represented by lines and figures,printed in brown. The brown lines, calledcontours, are lines of level each of which isdrawn at a stated uniform elevation above thesea. Fig. 1 shows an island as it mightappear if contours were marked on its sur-face. The lowest of these lines has every-where the same height, say 10 feet above thewater; the next line is 20 feet above thewater; the others are 30 feet and upward.Fig. 2 is a contour map of the same island.The 10-foot contour of the map marks out andsurrounds the part of the island rising morethan 10 feet above the water. The 20-footcontour shows two hills rising more than 20feet above the water and gives their positionand extent. The right-hand hill carries alsoother contours up to the 60-foot; it is there-fore more than 60 feet and less than 70feet high. Around the other hill runs the 80-foot level showing that it is more than 80 feethigh, and the absence of the 90-foot contourindicates that it is less than 90 feet. In theregion A the slope is gentle, and one mustwalk a considerable distance to ascend 10feet; the contours of the map are correspond-ingly far apart. In the region B the slopeis steep; one can ascend 10 feet in a shortdistance, and the contours of the map areclose together. Thus the contours by theirrelative closeness express the various slopesof the surface. They represent the positions,the forms and the heights of mountains, hillsand valleys.
The vertical space between two successivecontours is called the contour interval. Fora flat or gently undulating country a smallcontour interval is necessary; for a steep ormountainous country a large contour intervalis chosen. The smallest contour interval usedon the atlas sheets of the Geological Surveyis 5 feet. This is used for districts like theMississippi delta, the Dismal Swamp region,and the prairies about Chicago . In mappinggreat mountain masses like those in Colorado ,on a scale of ^^5, the contour interval is 250feet. For country of intermediate relief be-tween them other contour intervals of 10, 20,50 and 100 feet are used. The heights in feetabove mean sea level are given in brown figuresfor some prominent features, such as hill topsetc., and the different contours are distin-guished by numbers.
Drainage. —The watercourses are indicatedby blue lines, which are drawn full where thestream flows the year round, and are dottedwhere the channel is dry a part of the year.Where the stream sinks and reappears at thesurface, the supposed underground course isshown by a broken blue line. Marshes andcanals are also shown in blue.
Culture. —In the progress of settlement ofany region men establish many artificialfeatures. Of these roads, railroads and towns,names of natural and artificial details, andboundaries of towns, counties and states areprinted in black.
Culture changes as a region develops, andgradually comes to disagree with the mapwhich represents a previous condition. Henceit follows that the representation of culturerequires to be changed from time to time.Each sheet bears on its margin the date ofsurvey or of revision.
THE GEOLOGIC MAP.
This map represents the distribution ofrocks of different origins, of various ages, andof unlike compositions.
Rocks are of many kinds, and the extentof each kind occurring within the area of amap is shown by a printed device; hencethere is need of many devices, and these areobtained by printing different patterns in dif-ferent colors. To avoid confusion and make iteasier to read the map, rocks have been classi-fied, and a definite use of certain patterns torepresent certain groups of rocks has beendecided on.
Figure 3.—Stratified rocks resting on crystalline base.
According to their origin four groups ofrocks are defined for this purpose:
(1) Sedimentary rocks. —These are con-glomerate, sandstone, shale, and limestone,which, deposited beneath seas or other largebodies of water in successive beds or layersof mud, sand, gravel and shells or other cal-careous matter, have usually become hardenedinto rock. The areas of their occurrence atthe earth’s surface are shown on the map bycolors printed in patterns of parallel straightlines.
(2) Superficial deposits. —These are com-posed of* clay, sand and gravel. They occur inheaps and irregular layers, usually unconsoli-dated, and they have been spread on the sur-face of the land by water, by glaciers or bywind. Their distribution is shown on themap by colors printed in patterns of rounddots and circles.
(3) Altered rocks of crystalline texture .—These are rocks which have been so changedby pressure, movement and chemical actionthat the mineral particles have recrystallized;such a rock may have been originally sedi-mentary or igneous, but its character has beenaltered, and this alteration (metamorphism)may have proceeded so far that the origincan no longer be determined. Areas of suchrocks are shown on the map by colors printedin short lines irregularly placed.
(4) Igneous rocks. —These in a molten con-dition intrude among other rocks or flow fromcracks in the earth; they then form sheets,dikes and lava-flows; or they are thrown outby volcanoes and form ash rocks etc. Theirareas are shown by colors printed in angularfigures.
FORMATIONS AND PERIODS.
Century after century, from age to age ofthe earth’s history, rocks have been formed.The materials composing them and the condi-tions of their deposit have changed from timeto time and from place to place, and accord-ingly the kinds of rocks are varied. Wherebeds of sand were buried beneath beds of mud,sandstone may now occur under shale; wherea flow of lava cooled and was overflowed byanother lava, they may now be distinguishedone from the other. Each such mass of onekind of rock is limited in extent to the areaover which it was deposited, and is boundedabove and below by different rocks. It isconvenient in geology to call it a “ formation,”and to show upon the geologic map the occur-rence of different formations by printing allthe areas of each formation in one pattern ofthe same color.
The formations of stratified rocks often pre-serve the relative order in which they weredeposited, the younger upon the older. Theymay contain fossils, remains of creatures,many of w T hich existed only during certainperiods. Thus where strata are not toogreatly disturbed, their relative ages may bedetermined by their positions one above theother; or fossils may show the order in whichupheaved strata once lay, or the similar ageof widely separated strata. To show thesefacts on the map, the history of the sedi-mentary rocks is divided into nine periods,and a color is assigned to each period. Each.period is further distinguished by a lettersymbol, so that the areas may be recognizedwhen the colors, on account of fading, colorblindness, or other causes, are not distinct.The names of the periods in proper order (fromnew to old) with the color and symbol as-signed to each, are given below: