New Holland does not show any signs of recent volcanic activity, except at its most southern point (Australia Felix), at the foot of the Grampian Mountains. N.W. from Port Philip, as also towards the Murray River, there are numbers of volcanic cones and sheets or flows of lava.
It would be of great interest and utility to observe the relative inclinations of the Magnetic and the Geographical Equators, by means of the dip of the magnetic needle, though this will be rendered more difficult, from the fact of the ship's course being easterly, that is, contrary, to the Equinoctial current. As regards the low temperature of the current, which I discovered in 1802, running up from 40° S. to the Gallipagos along the coast of South America, and then turning westward, it would be highly important to investigate whether in the eastern part of the South Sea in 7° N. and between 117° and 140° W., there really exists in every season a counter current from west to east. But I need not enlarge upon this topic to such attentive navigators.
The line of no inclination was crossed six times by Duperrey between 1822 and 1825. When I first discovered, near Truxillo, the low temperature of the cold Peruvian current, it was 12°·8 Réaumur (60°·8 Fahr.). The temperature observed in the course of twenty years by Mr. Dirckinck von Holmfeld, in the neighbourhood of Callao, expressed in degrees of Réaumur, were as follows:—
| September 1802 | 12°·8 | (Fahr. | 60°·8) | Thermometer in the air. 13°·3 Réaumur. (61°·92 Fahr.) |
| November 1802 | 12°·4 | " | 59°·9 | |
| December, end of | 16°·8 | " | 69°·8 | |
| January 1825 | 12°·7 | " | 60°·57 | |
| February 1825 | 15°·3 | " | 66°·42 | |
| March 1825 | 15°·7 | " | 67°·32 | |
| April 1825 | 14°·5 | " | 64°·62 |
The temperature of the sea I found to be 22° Réaumur (81°·5 Fah.) north of Cape Blanco, when on my way from Callao de Lima, at which point the cold current diverged towards the Gallipagos.
Between the Gulfs of Guayaquil and Panama, north-east of the cold current, the temperature of the sea during the month of April rose as high as 24°·5, (87°·12 Fahr.). Within the range of the current, Mr. Dirckinck had carried on his observations in compliance with my instructions, by means of thermometers that had been compared by Arago. Everywhere in the current, in December 1824, he found from 16° to 18° (68° to 72°·5 Fahr.); between Quilca and Callao, in January, 1825, from 18° to 19° (72°·5 to 74°·75 Fahr.); between Chorillos, near Lima (Lat. 12° 39′ S.) and Valparaiso, in August, 1825, from 13°·8 to 10°·5 (63°·05 to 55°·62 Fahr.); between Chorillos and San Carlos de Chiloe, in June, 1825, from 18°·8 to 9°·2 (74°·3 to 52°·7).
In sailing from the Sandwich Islands to the west coast of America, the Imperial Expedition will have to choose between the Ports of San Francisco or Acapulco. The first choice would be of great mineralogical advantage for those regions of the United States, lying North of the river Gila.[3] Parallel with the chain of the Rocky Mountains, which, according to Marcou, contains up to the present day several volcanoes in full activity in its northern part (Lat. 46° 12′ N.), run single, and at certain points double ranges of coast chains from San Diego to Monterey, from 32° 15′ N. to 46° 45′ N. They begin with the coast range specially so-called, which is a continuation of the high ridge of the Peninsula of Lower or Old California; after which, farther to the North, there follow in succession, first the Sierra Nevada di Alta California, between 36° and 38° N. the lofty Shasty mountains, and the Cascade Range, nearly twenty six miles distant from the littoral, including many high and active volcanoes, and extending far beyond Fuca Straits. The following are still in eruption:—Mount St. Elias (46° 2′ N.); Mount Regnier, or Rainier, (46° 46′); and Mount Baker, (48° 48′.) These three active cones would be most conveniently visited by the geologist of the expedition from San Francisco, as would likewise the whole Cascade Range. We have as yet no certain intelligence as to the geology of the entire longitudinal auriferous valley of the Sacramento River, (where a trachytic crater, in a state of disintegration, is known as the Butt of Sacramento). Does the auriferous quartz occur in veins, and are these still in situ, or are they broken up? What description of rock is traversed by these veins? Does the wash-gold here contain occasionally, as in the Ural Mountains, fragments of vein-stones with isolated cavities, in which are found impressions of leaves and membranes, clearly proving that they have not been rolled, or transported by water, any great distance to the spot they now occupy? Have these been found, alongside of gold, diamonds, platinum, osmium, iridium, or mercury?
[3] The Gila falls into the Colorado about forty miles above the embouchure of the latter into the head of the Gulf of California.
Should the frigate steer for Acapulco, it may be assumed that there exists an intention to cross the Continent to Mexico and Vera Cruz, from the volcano of Colima (1877 toises) as it were, along the parallel of the range of volcanoes, and greatest heights rising in detached groups between the two seas, about the parallel of 19° N. New astronomical observations are greatly needed for determining the position of the volcanoes of Colima and Jorullo (667 toises). The volcano of Colima, with its twin peaks de fuego and de nieve, should be carefully examined, as also the volcano of Jorullo, with the fragments of granite enclosed in its lava; the Nevado de Toluca (2372 toises), Popocatepetl (2772 toises), Itztaccihuatl (2456 toises), Cofre de Perote (2098 toises), and the volcano of Tuxtla (18° 28′ N.), on the eastern slope of the Sierra St. Martin, from which a column of flame shot up with great violence on 2nd March, 1793, a fair specimen of what the Spaniards term Malpays, the Sicilians Sciarra viva. The face of the country is covered over with boulders of lava, at San Nicolas de los Ranchos, at the foot of Popocatepetl, adjoining the city of Puebla de los Angeles, after which, on the road from Puebla to Vera Cruz, will be observed two narrow strips of boulders of cooled basaltic lava, rich in olivine. Similar examples will be found at Parage de Carros, near Tochtilacuaja and Loma de Tablas, between Cancas and the Casas de la Hoja. The mere ascension of volcanic cones is geologically of far less importance, than the bringing away numerous specimens, carefully selected, of various trachytic rocks, which, by their oryctognostical composition, are characteristic of each volcano. I would nevertheless recommend that the Pico del Fraile of the Toluca volcano (2372 toises) should be ascended, proper caution being used. From this very sharp peak, I brought away thin plates of trachyte perforated by lightning, and within the holes of a melted glassy surface, resembling those brought from Little Ararat. Both for the miner and geologist, an interesting and useful visit might be paid to the rich mines of Guanaxuato and the Mines de la Biscaina and Regla, on the road from Mexico to Real del Monte, so as to observe the close connection subsisting between the richer silver ores, occurring in trachytic porphyry without quartz, but with felspar, (glassy felspar?), and the thoroughly volcanic Cerro del Jakal, abounding in obsidian, and the Cerro de las Navajas (Razor Range), which remind one of the environs of Schemnitz, with the sole exception, that the trachytes "porphyres meulières" of Beudant, are wanting here.
As it is highly desirable that considerable time should be devoted to the volcanoes of Quito, Peru, and Chili, it appears uncertain whether the course of the frigate, on leaving Acalpulco, will be shaped direct for Guayaquil, thus reversing the route taken by myself, or whether she will not touch at some of the central American ports—Realejo or Sonsonate. The crowded series of volcanoes in Central America, of which no less than eighteen, conical or dome-shaped, may be considered as still in active eruption, would yield a rich harvest of facts of all kinds in elucidation of the theory of volcanic action, such as have never hitherto been sufficiently taken advantage of. We are still in need of the mineralogical determination of the rocks, while the form and situation of the mountain masses have been well described by Squier, Oersted, and other modern travellers. The greater number, indeed, of the eruptions of scoriæ and slag were unaccompanied by streams of lava, as, for example, those of Mount Isalco, abounding in ammonia. But recently eye-witnesses have furnished us with quite different accounts regarding these eruptions, in the case of several volcanoes—as the Nindiri (a twin volcano with that called Massaya), on which Dr. Scherzer has lately shed much light; the Volcano el Nuevo, erroneously called Volcano de las Pilas, that of Coseguina, situated on the Great Bay of Fonseca, and that of San Miguel de Bosotlan, from which there flowed an extensive stream of lava in July 1844. It would be most tempting to pass by land from Mexico southwards to Oaxaca, and thence to the Isthmus of Guasacualco or Tehuantepec, and Chiapas, so as to rejoin the frigate at Realejo or Sonsonate. Facts might be obtained, in such a journey, of great value in determining the dependence of geological phenomena on each other; but it is to be feared it would be attended with too much fatigue and loss of time. For similar reasons, it cannot be proposed that the scientific gentlemen attached to the Expedition, should leave the frigate for three or four months, when they reach Central America, in order to cross by rail the Isthmus of Panama, with the object of examining the Volcancitos of Turbaco and Gabra Zamba, both active, and thence ascend the Rio Magdalena from Carthagena de las Indias, as far as Honda, whence they could proceed by Bogotà and Popayan to Quitó.