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the direction in relation to the great circles of the sphere. Physical maps in vertical projections can only be constructed on knowing, for the points through which the plan of projection passes, the three coordinates of longitude, latitude, and elevation above the level of the ocean; and it is only in uniting barometrical measurements with the results of astronomical observations, that the section of a country can be drawn. This kind of projection will become more frequent in proportion as travellers shall addict themselves more assiduously to barometrical observations. But few provinces of Europe at this day offer the necessary materials for constructing views analogous to those published by me of equinoxial America. The construction of the sections, plates v.1, vis, viii, are absolutely uniform. The scales are the same in all the three views; the scales of distance are to those of height nearly as one to twenty-four. The three maps indicate the nature of the rocks which compose the surface of the soil. This knowledge is interesting to agriculturists; and it is also useful to engineers employed in constructing roads or canals. I have been blamed for not exhibiting in these sections the superposition or situation of the secondary or primitive strata, their inclination or their direction. I had particular reasons for not indicating these phenomena. I possess in my 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 entre 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 v1.1.) 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 endeavoured to improve
them, will feel, like myself, 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 distinguish d 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 important 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 Alps and 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 v11 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 v1.1 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