itineraries all the necessary geological materials for forming what are usually called mineralogical maps. A great number of these materials were published by me in my recent work on the measurement of the Cordillera of the Andes; but on mature examination I adopted the resolution of separating entirely the geological sections which display the superposition of rocks from the physical views which indicate inequalities of surface. It is very difficult, I had almost said impossible, to construct a geological section of an extensive country, if this section must be subjected to a scale of elevation. A stratum of gypsum of one metre* thick is often more interesting to a geologist than an enormous mass of amygdaloid or porphyry; for the existence of these very slender strata, and the manner in which they lie, throw light on the relative antiquity of formations. How then shall we trace the section of entire provinces, if the magnitude of the scale is to be such as to exhibit masses so inconsiderable? How shall we indicate in a narrow valley, in that of Papagayo, for example, (Plate vII.) in a space of onet or two millimetres of breadth, which the valley occupies in the drawing, the different formations which repose on one another? Those who have reflected on graphical methods, and

*39.371 inches. Trans.

↑ A millimetre contains .03937 of an inch. Trans.
VOL. I.
I

endeavoured to improve them, will feel, like my. self, that these methods can never unite every advantage. A map, for instance, overcharged with signs, becomes confused, and loses its principal advantage, the power of conveying at once a great number of relations. The nature of the rocks and their mutual superposition interest the geologist much more than the absolute elevation of formations and thickness of strata. It is sufficient if a geological section expresses the general aspect of the country, and it is only in freeing it from scales of height and distance that it can indicate luminously the phenomena of stratification, which it is of importance for geologists to know.

The physical view of the eastern declivity of New Spain is composed of three sections, which I have distinguished by different colours. The cities of Mexico, and la Puebla de los Angeles, and the small hamlet of Cruz Blanca, situated between Perote and las Vigas, are the points in which the intersection of the three planes of projection is made. I have added the longitude and latitude of these points, the medium direction of each section, and its length in French leagues of twenty-five to the degree.

The two great volcanos on the east of the valley of Tenochtitlan, the Pic d'Orizaba, and the Coffre de Perote, were placed in the drawing according to their true longitudes. We have represented them as they appear when a thick fog covers their

base, and when their summits are seen above the clouds. Notwithstanding the enormous breadth of these colossal mountains, we have not dared to represent their whole contours, on account of the great inequality of the scales of height and distance. These volcanos would have disfigured the view, rising like so many slender columns above the plain. I have endeavoured to represent very exactly the strange form, I had almost said the particular physiognomy, of the four great mountains of the Cordillera of Anahuac; and I flatter myself that those who travelled from Vera Cruz to Mexico, and who have been struck with the wonderful aspect of these majestic mountains, will perceive that the contours are exhibited with precision in this plate, and in No. 1x and x.

That the reader may fix in his mind some im portant facts of physical geography, we have marked on the two sides of the views, near the scales of elevation, the height of the Chimborazo, and of several mountains of the Alpsand Pyrenees; that of the limit of perpetual snows under the equator, under the parallel of Quito, and the 45° of latitude; the middle temperature of the air at the foot and on the slope of the Cordilleras; and lastly, the elevations at which certain Mexican plants begin to be seen, or cease to vegetate in the mountainous part of the country. Several of these phenomena are even repeated in all the maps; a repetition analogous to what all the thermometer

scales formerly exhibited, which indicated, though very inaccurately, the maximum and minimum of temperature observed under such or such a zone. I believed that these sections, which have some analogy with the large view in my Geography of Plants, might perhaps contribute to propagate the study of the natural history of the globe.

VII. PHYSICAL VIEW OF THE WESTERN DECLIVITY OF THE TABLE-LAND OF NEW SPAIN.

This and the preceding view, and the section of the valley of Tenochtitlan (Plate VIII.) are drawn up all three according to the principles laid down by me in discussing the section of the eastern slope of the Cordilleras. I have framed on the same scale Plates VII and VIII, that they may all be united at pleasure into one, which will then extend from the Atlantic Ocean to the South Sea, and which will develope to the geologist the extraordinary conformation of the whole country.

It may be necessary to observe to those who wish to unite the sections VII and vIII, in cutting the two vertical scales on which the heights of Puy-de-Dôme and Vesuvius are marked, that the planes of projection of these sections intersect each other almost at right angles, in the centre of the city of Mexico. The medium direction of the first section, which is itself composed of different

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 barometrical 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. VII, 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 Navidad, is far from giving a juster idea of the geological consti

*6560 feet, Trans.