Geology and Mineral Resources of Paraguay A Reconnaissance

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GEOGRAPHYbisects the Republic of Paraguay in its lower reaches,and marks striking differences in topography, vegetation,and cultural development between the two partsof the country. It has a total fall of only 250 meters inits 2,500 kilometer course, and a speed through itsParaguayan portion of 5 to 7 kilometers per hour. Itis several hundred meters wide in most places and isgenerally navigable as far as Corumbd, Brazil, by shipswith 3-meter draft and even farther by smaller craft.Its course is characterized by innumerable meandersand by frequent channel shifts. From the Rio Apasouthward almost to Conception the east bank ismarked by discontinuous cliffs of limestone and marble,nearly vertical, and several tens of meters high inplaces. Elsewhere the sandy to muddy banks slopegently into the river or form steep cutbanks, carvedfrom flood-plain alluvium and only a few meters highat most.The Rio Paraguay, with its tributaries, drains all butthe southeastern portion of the country. The RioPilcomayo, heading 2,000 kilometers westward in theAndes in Bolivia, and forming much of the internationalboundary with Argentina, is the largest tributary 011the western or Gran Chaco side of the river. It isnavigable in places by small boats. Except for itslength and relative continuity, it is similar to any ofthe other streams that drain the Gran Chaco. Normally,they are all sluggish, intermittent, and discontinuousstreams that wind their way slowly across thenearly level plains between banks that are seldom morethan a couple of meters high. During rainy seasonsthey overflow their banks and spread over most of theland. Working together, first in one part of the GranChaco and then in another, they are still building upthe plains. Their loads of mud, silt, and fine sanddebris, earned down from the Andes, are redistributedover the plains during periods of overflow or by changesof course that take place on most streams, if not fromyear to year, at least from decade to decade.Most of the eastern part of Paraguay drains into theRio Paraguay through seven main tributaries andseveral smaller ones. Five are 180 to 275 kilometerslong and are navigable in part by small boats and barges,but all the tributaries are slow and shallow in mostplaces and subject to frequent shifts in channel.The Rio Alto Parana, about 3,000 kilometers long,heads deep in southeastern Brazil; in its lower reachesit forms the southeastern boundary of Paraguay for adistance of 850 kilometers. On its way toward Paraguayit meanders slowly across high basalt plains, butwhere it first touches Paraguay it plunges abruptlyover a series of jagged basalt escarpments to form theawe-inspiring cataracts of Salto del Guaira. Fromthese falls to Encarnaci6n it is incised in basalt andfollows a relatively straight, narrow and swift courseas compared with its upstream or downstream portions.From Encarnaci6n to its junction with the Paraguaythe stream is broad and shallow, characterized byshifting, braided channels. It is navigable by moderatesizedcraft as far upstream as Guaira. The southeasternpart of the country drains to the Alto Parandby means of 11 major tributaries, only 2 of which aremore than 150 kilometers long. These, too, are slowmeandering streams and resemble the tributaries ofthe Rio Paraguay in all but two important respects.First, though their meanders superficially resemble theaimless wanderings of the rivers farther west, on closerstudy they are seen to be related to fracture patternsin the basalt flows that underlie most of the area.Second, most of the tributaries make their final descentinto the Alto Parana gorge by a series of low steplikefalls or rapids where they cross the edges of the basaltflows. Recognition of these two features is of greathelp to the reconnaissance geologist. Moreover, thefalls seem to represent the only potential source ofhydroelectric energy that is not subject to internationalcomplications.There are two fairly large permanent but shallowlakes in the country Lago Ypoa and Lago Ypacaraiand countless intermittent ponds, lagoons, and swamps,some of them enormous.CLIMATEThe climate, comparable in some respects to that ofsouthern Florida or south-central Texas, ranges fromsubtropical to tropical. Paraguay is an interior countryof hot summers and mild winters, with the dominantcharacteristic of the weather extreme unpredictabilityfrom month to month and from year to year.The winter months, especially July through Septemberare generally somewhat drier and considerably colderthan the summer months, December through February,with rainfall ranging between one-fifth and two-thirdsthe average of all months.The summer months and shorter periods throughoutthe year are hot and humid, though moderated fromtime to time by heavy rains that may last only a fewminutes or may continue almost steadily for severaldays. Virtually all living habits, from housing arrangementsto business hours, are adjusted to the predominantlywarm parts of the year. This is particularlynoticeable during the comparatively few cooler days ofwinter, when tile floors, high ceilings and the generallack of heating arrangements tend to make temperaturesof 5° to 10°C (41° to 50°F) somewhatuncomfortable.Figure 3 shows the geographic distribution of meanaverage temperatures by isotherms ranging from 22° to
8 GEOLOGY AND MINERAL RESOURCES OF PARAGUAY24° C (72° to 75° F). The average maximum summertemperature measured during the same period, 1940-50, shown on figure 3 was 36°C (97°F) in the northernGran Chaco and the average minimum wintertemperature at Encarnacidn was 10°C (50°F).During this period, an extreme maximum of 43°C(110°F) was recorded in the Gran Chaco in July1942, and extreme minimums of 4° and 5°C (21°and 23°F) were measured at Encarnacion in July1945 and in the Gran Chaco in July 1942. Five ofthe country's eight weather stations reported freezingtemperatures or below in at least 1 month during theperiod 1940-50, and Encarnacidn had freezing weatherin one or more months in each of the 11 years.Figure 3 shows also that the average annual rainfallduring the period 1940-50 inclusive ranged uniformlyfrom 600 millimeters (23 inches) in the western GranChaco to 1,800 millimeters (71 inches) along the easternborder of the country. This uniform, and ratherabrupt, increase in rainfall from west to east bears noapparent relationship to the local topography, hence isobviously controlled by more regional factors. Of23Average annual temperature,in degrees centigradeFIGURE 3. Average annual rainfall and temperatures in Paraguay, 1940-50, inclusive.Compiled from various sources, 1953, by Dorothy A. Oassel for the Instituteof Inter-American Affairs.these, the high Andes on one side and the much lowerhighlands of southern Brazil on the other, together withthe prevailing north winds, appear to be dominant incontrolling rainfall.The periods of highest relative humidity are generallyin March, June, July, and August; the lowest occurduring December in eastern Paraguay and in Augustand September in the Gran Chaco. Evaporation generallyexceeds precipitation from June through Augustin the east and during nearly all of the year in theGran Chaco.VEGETATIONMore than half of Paraguay is heavily forested withvirgin stands of tropical and subtropical trees andshrubs. More than 3,000 species of these have beenrecorded. Many of the trees are deciduous hardwoods,but new leaf growth follows so quickly on the fall ofolder leaves, or even accompanies it, that bare trees arerare at any season. Many species bloom profusely inthe spring and a few bloom at other seasons, lendinggreat masses of white, yellow, pink, purple, and bluecolors to the landscape. The undergrowth of shrubsand vines is commonly thick, and virtually impenetrablein places, especially around the fringes of theforested areas. There is no true rain forest, but hereand there many individual trees tower far above theforest blanket.In eastern Paraguay, the forests once covered nearlyall of the hills and rolling uplands. In the more heavilypopulated central region, the hills, called "islas" or"islands," have been partly or entirely cleared andnow support the bulk of the country's farm lands.Farther from centers of population the forests are stillvirtually untouched, though here and there they arestrikingly marked by angular farm clearings, most ofthem small in comparison to the forests around them.The forestry industry itself, large though it is, does notgenerally result in denudation of the land or in clearingsthat can be converted to agriculture. This is becausethe trees most sought by lumbermen are widely scatteredand their cutting results in little or no visiblethinning of the forest.The lower and flatter portions of eastern Paraguay,called "campos" or "camplands," are grass covered inlarge part, with trees and brush in thick borders alongthe water courses. Parts of these plains, which areused principally for grazing, are swampy, but somecould be easily drained by ditching.On the Gran Chaco side of the Rio Paraguay, vastareas of nearly level grasslands drain into sluggishmeandering rivers that are bordered by thick stands ofdeciduous trees. The grasslands are interspersed withpeculiar brush forests of mesquite and many other
Page 1 and 2: Geology and MineralResources of Par
Page 3 and 4: UNITED STATES DEPARTMENT OF THE INT
Page 5 and 6: IVCONTENTSMineral resources._______
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Page 10 and 11: INTRODUCTION(1947, 1952) are the on
Page 12 and 13: GEOGRAPHYFIGURE 1. Index map showin
Page 16 and 17: GEOGRAPHY 9thorny semidesert plants
Page 18 and 19: GEOLOGY 11chiefly logs, rough lumbe
Page 20 and 21: IGNEOUS AND METAMORPHIC ROCKS 13and
Page 22 and 23: 15FIGURE 7. Arkosic sandstone (spec
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Page 32 and 33: 25FIGURE 18. Ehyolite (specimen P-3
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56 GEOLOGY AND MINERAL RESOURCES OF
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60lower third of the series consist
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66possible Cretaceous age. Baker (1
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174 GEOLOGY AND MINERAL RESOURCES O
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MINERAL RESOURCES 87a pivot, show s
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MINERAL RESOURCES 89of the bright r
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Geology and Mineral Resources of Paraguay A Reconnaissance

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