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planes, is from east to west; the medium direction of the second, the road from Mexico to Acapulco, is from S.S.W. to N.N.W. The prolongation of the first section would extend nearly by Pascuaro and Zapotlan, to the Villa de la Purificacion. This plane prolonged to the west would terminate on the shores of the South Sea, between Cape Corrientes, and the port de la Navidad. As New Spain swells out singularly in this western direction, it would follow that the descent of the Cordillera, from the valley of Tenochtitlan to the plains of the intendancy of Guadalaxara, would be twice the length of the road from Mexico to Acapulco, sketched in plate vii. The balometrical measurements which I made between Valladolid, Pascuaro, Ario, and Ocambaro, prove, that in tracing this transversal section in the direction of the parallels of 19 or 20 degrees, the central plain would preserve the great elevation of 2000" metres for more than sixty leagues to the west of the city of Mexico, while, in the direction of the section, No. v1.1, the plane never reaches this elevation, after leaving the valley of Tenochtitlan towards the S.S.W. Yet a section directed from east to west, from Vera Cruz to the small port de la Navi, ad, is far from giving a juster idea of the geological constitution of New Spain than the reunion of my two sections, No. vii and v1.11. A simple consideration of the true direction of the Cordillera of Anahuac is sufficient to prove what I advance. The central chain of the mountains runs from the province of Oaxaca to that of Durango, from the S.E. to the N.W.; consequently, the plane of projection, to be perpendicular to the longitudinal axis of the Cordillera, should not be placed parallel to the equator, but drawn from the N.E. to the S.W. By reflecting on the particular structure and limits of the groupe of mountains, in the neighbourhood of the capital of Mexico, we shall find that the reunion of the two sections, No. vii and viii, gives a less imperfect representation of the conformation of the country than we should be tempted to believe from purely theoretical ideas. In this mountainous region between the 19° and 20° of latitude, nothing announces a longitudinal crest. There are none of those parallel chains which geologists always admit in their works, and which geographers represent in the most arbitrary manner, in their maps of the two continents, like ranges of elevated dikes. The Cordillera of Anahuac increases towards the north, from whence the inclined planes formed by the eastern and western declivities are not parallel to one another in their middle direction. This direction is almost N. and S. along the coast of the gulf of Mexico, while it is S.E. and N.W.

* 6560 feet. Trans.

l

in the declivity opposite the Great Ocean. Hence the sections, to be perpendicular to the lines of declivity, cannot be in the same plane of projection.

VIII. PHYSICAL VIEW OF THE CENTRAL TABLE-LAND OF NEW SPAIN.

The section of the road leading from Mexico to the mines of Guanaxuato, the richest of the known world, was drawn up under my eye at Mexico, by M. Raphael Davalos", a pupil of the school of mines, and a very zealous young man. This drawing displays to the naturalist the great elevation of the table-land of Anahuac, which extends to the north much beyond the torrid zone. The extraordinary configuration of the Mexican soil recalls the elevated plains of central Asia. It would be interesting to continue my section from Guanaxuato to Durango and Chihuahua, particularly to Santa Fe in New Mexico. For the tableland of Anahuac, as we shall hereafter provet, preserves towards the north for an extent of more than two hundred leagues more than 2000", and for an extent of five hundred leagues more than 800f metres of absolute elevation.

* M. Davalos, as well as M. Juan Jose Rodriguez, a native of the Parral, in the provincias internas, and well informed in physical science, were so good as to assist me for several months in the construction of a great number of geological maps which will be afterwards published. I am pleased to have an opportunity of giving a public testimony of my gratitude to gentlemen so distinguished for their talents and application.

t Book I. and book III.

IX. PICTURESQUE VIEW OF THE VOLCANOS OF MEXICO OR LA PUEBLA.

This plate and the immediately following one were destined at first to appear in the physical atlas, which will accompany the historical account of my travels in the equinoxial regions. I mean to unite in that atlas such sketches as will show the physiognomy of the colossal summits which crown the ridge of the Cordilleras, and form as it were their crest. I thought that these contours, compared with those in the excellent itinerary of M. Ebel, or the beautiful drawings of M. Osterwald, might prove interesting to the geologists who wish to study comparatively the Alps of Switzerland, and the Andes of Mexico and Peru. Though the object of the work which I now publish is more to describe the territorial riches than the geological constitution of New Spain, I have thoug't proper to add to the Mexican atlas the picturesque views No. 1x and x, to serve as a supplement to the map of the valley (Plate iii.), and to give a more lively idea of the beauty of the situation of the city of Mexico. These same summits, the Popocatepetl and the Citlaltepetl, the first of which is visible at Mexico

* 6560 feet. Trans. + 2624 feet. Trans,

and Cholula, and the second at Cholula and Vera Cruz, served me to verify the meridian difference of the city of Mexico and the port of Vera Cruz by a method very advantageous, but hitherto little followed; that of perpendicular bases, azimuths, and angles of altitude”. The city of Mexico is nearer by one half to the two Nevados de la Puebla than the cities of Bern and Milan are to the central chain of the Alps. This great proximity contributes much to give an awful and majestic aspect to the Mexican volcanos. The contours of their summits, covered with eternal snow, appear so much the more marked, as 'the air through which the eye receives the rays is more rare and transparent. The snow is of a most extraordinary brilliancy, particularly when, it descends from a sky of which the blue is always deeper than that of the sky which we see from our plains of the temperate zone. The observer finds himself, in the city of Mexico, in a stratum of air, whose barometrical pressure is only 585 millimetress. It is easy to conceive, that the extinction of light must be very trifling in an atmosphere so little condensed, and that the summit of the Chimborazo, or the Popocatepetl, seen from the plains of Riobamba or Mexico, must exhibit

* See above, p. xxiii. and my Recueil d'observations astronomiques, vol. I. p. 373. * Nearly twenty-three inches. Trans,

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