The attached letters Ws., Br., and Ht., indicate the barometric measurements of Dr. Wislizenus, Obergrath Burkart, and myself. To the valuable memoir of Dr. Wislizenus there are appended three profile delineations of the country; one from Santa Fé to Chihuahua over Passo del Norte; one from Chihuahua over Parras to Reynosa; and one from Fort Independence (a little to the east of the confluence of the Missouri and the Kanzas River) to Santa Fé. The calculation is based on daily corresponding observations of the barometer, made by Engelmann at St. Louis, and by Lilly in New Orleans. If we consider that in the north and south direction the difference of latitude between Santa Fé and Mexico is more than 16°, and that, consequently, the distance in a direct meridian direction, independently of curvatures on the road, is more than 960 miles; we are led to ask whether, in the whole world, there exists any similar formation of equal extent and height (between 5000 and 7500 feet above the level of the sea). Four-wheeled waggons can travel from Mexico to Santa Fé. The plateau, whose levelling I have here described, is formed solely by the broad, undulating, flattened crest of the chain of the Mexican Andes; it is not the swelling of a valley between two mountain-chains, such as the “Great Basin” between the Rocky Mountains and the Sierra Nevada of California, in the Northern Hemisphere, or the elevated plateau of the Lake of Titicaca, between the eastern and western chains of Bolivia, or the plateau of Thibet, between the Himalaya and the Kuen-lün, in the Southern Hemisphere. IDEAS FOR A PHYSIOGNOMY OF PLANTS.