granite gate. This was once the passage of the tidal currents between the Atlantic and the Pacific. The protrusion of eruptive masses skirting the base of the California Cordilleras seem to have been checked here by some means; their occurrence is at least less frequent in this neighborhood. A little further north, however, another shoot of igneous and metamorphic rocks abuts against the western granite walls. They are a northwestern continuation of the Sonorian Gulf Sierras, crossing the Colorado Valley in the vicinity of the mouth of the Gila, and joining the California Cordilleras somewhat to the north of the before mentioned mountain pass. Along the eastern base of the Sierra Nevada we find volcanic activity again fully developed. It not only skirts these walls of primary and metamorphic rocks, but seems to have its theatre over the whole area between the Rocky Mountains and the Sierra Nevada. According to the accounts of American and other explorers, this vast area abounds in salt lagoons, soda lakes, solfataras, geysers, warm and hot springs of various character, extinct craters, and other traces of wide-spread volcanoism. Tertiary deposits seem not to be wanting throughout these regions; some of them are proved beyond doubt to belong to the Miocene age. Numerous fossils, verifying such conclusions, have been collected by various persons connected with government expeditions. Among other discoveries, is one of the highest importance; this is, the existence of a fossil shell (Cardita planicosta) on the Pacific slope of the Sierra Nevada. This shell, originally belonging to the Paris basin, is common also on the Atlantic side of this continent, and is now known to occur also on the Pacific. From this we have the proof of a former immediate connexion in these latitudes between the two oceans of our globe. Without making any further mention of other numerous geological and palæontological facts relating to those regions which have been brought to light since the last decenium, we consider the area from the present head of the Gulf up to the Great Salt Lake basin as one tertiary, if not quaternary sea. There were, besides the present Gulf, other inlets from the Pacific to this interior sea; some of them we know already, and others, no doubt, will be discovered a short time hence. However wide this inland sea may have extended, we find on its western shore primary and metamorphic rocks lined by tertiary strata, and on the east shore metamorphic and volcanic rocks prevail. The bottom of this present waste, thickly overlaid with diluvial deposits, seems to have been thrown out of its level by upheaving forces from below. We may call them pluto-volcanic, employing a term to designate the immediate ejective forces and the upheaving motions of a general character. In the regions before us we have innumerable traces of ejected masses, in the shape of igneous dykes and sierras of similar petrographic character, but varying in size. We find, in fact, the former horizotalism of the whole Gulf basin along the Rio Grande and its tributaries, everywhere traversed, intercepted and broken up by metamorphic and igneous mountain ranges. It seems that the observer, in no other locality, stands in closer presence of these very pluto-volcanic upheaving forces, than on the western edge of this ancient inland sea. Whoever passes over this ground, particularly the desolate scenery about Carrizo Creek, if his mind should be the least open to impressions of this kind, must be struck with awe! He will find himself in a locality where nature gives, in a few bold words, a whole sentence of her cosmogonic history. Here we everywhere find distinct marks of metamorphism, the result of a closely allied cyclopean and neptunic activity. Changes in the physiography of the country.-One fact closely connected with the physiography of these desert regions is worthy of notice. The regions around New River and other beds of drainage, are inhabited by Indians, who raise on those alluvials, pumpkins, melons, and reap the seed of certain grasses and seeds, especially of an "Amaranthus," called by the Mexicans Quelite," and by the Americans "Careless weed." These plants are dependents of alluvial soil. At present the New River is often subject to dry up entirely for one or two successive seasons, thus leaving this forlorn people to the most bitter adversity. Whether this was always the case we were not able to find out, but there is some probability such was not, for if it had been so, these Indians or their ancestors would not have settled in this neighborhood. According to traditions and observations made on the spot, water must have been distributed in former times more abundantly over the desert west of the Colorado, since either changes in the conditions of the climate or alterations in the level of the land, or both, must have taken place, causing a gradual diminution of drainage, and necessarily a subsequent decrease of population. All accounts which have come to our knowledge agree upon an increased sterility of soil. Several localities are pointed out where in former times plenty of grass and mezquite, besides spacious planting grounds, were found. Pascual, the present chief of the Yuma Indians, when a child, lived at " Alamo Mocho," where at the time (he is now about 60 years of age) water was running constantly, as it did also at New River. The place of Pascual's childhood was called Hu-ta-pil, because plenty of tunas (the fruit of opuntia) grew there. Of all this, nothing seems to have been left but the name " Alamo Mocho,'' (stunted cotton-wood,) as one tree of the kind marked this locality long after water had ceased to run here. If such changes really have taken place, and there is hardly room for doubt, we are inclined to ascribe the cause to an alteration of level-hyetographical changes being, perhaps, but the result. It is not improbable that this portion of the Peninsula participated in a similar rising as Clavijero mentioned in regard to the southern regions of this country. We have already mentioned the earthquakes to which these regions, together with all California, are subjected. They may be considered as principally instrumental in producing those general changes upon the surface. Here every three or four years heavy vibrations of earthquakes are witnessed. Major Heintzelman thus writes of a severe shock experienced the 29th of November, 1852: "At the time the river was unusually low, and (the Laguna) behind the post uncommonly high. (Behind Camp Yuma there is an old river bed.) Low grounds became full of cracks, many of which spouted out sulphurous water, mud, and sand. Further below, the river bed was changed considerably. The re-opening of a salfatara in the southwest corner of the desert was mentioned as the result of this earthquake." Similar accounts of the same event were given by another eye-witness. He was at the time on board a small river steamer, about twenty miles below the mouth of the Gila, and on guard, when a heavy shock was felt on board, upon which the general alarm was given, "Boat aground!" Our informant, formerly a sailor in the South Pacific, on the coast of Peru, and also a visitor of the Sandwich Islands, having witnessed there the outbreak of the notorious "Ruaroa," knew instantly what it was, and coolly remarked, touching bottom with a soundingpole: "Yes! "Yes! Boat aground in eight feet water!" The waters of the river were thrown in a sort of boiling motion, with a strongly rippled surface. The river banks on one side caved in; and on the other separated in a thousand cracks, from which dust, sand, mud, and water was jetting. In front of the steamer, at the time, there was a ferry-boat, loaded with sheep, just crossing the river. The hands in charge of it not knowing what to think of the phenomenon, in their fright threw away their oars and poles, and held on to the sides of the boat, expecting to go down. The river formed new bends, leaving portions of its old bed so suddenly that thousands of fishes were left lying on the muddy bottom to infect in a few days the air along the river by their putrefaction. The frequency of earthquakes occurring here forms also a point in the mythology and traditional tales of the aborigines, which has been referred to elsewhere. Mountain rupture.-Eight miles below Fort Yuma another trace of the action of earthquakes is exhibited on the eastern foot of the Sierra Culaya, or Pilot Knob, as it is styled by the Americans. The metamorphic rock forming the knob is a dark syenitic granite, with much hornblende. At its base the Colorado turns abruptly to the south. One of the outrunners of the sierra shows a rupture with an average width of about thirty feet. The edges of this mountain cleft fit each other, so as to leave no doubt that they formed one mass. By means of this gap the post-tertiary banks of the river, 70 feet high, can be seen. (See annexed sketch.) The course of the Colorado, which runs on higher ground than the surrounding desert, and the configuration of the junction of the Gila and Colorado, (elsewhere described,) is, perhaps, another result of a geological disturbance in the general level of country. Dr. J. Leconte, in his notes on some volcanic springs in California, which have been heretofore referred to, mentions also the anomalous course of the Colorado not taking the lowest level of the desert, but retaining its bed about 130 feet above it. He ascribes the cause to the deposition of its sediment, somewhat like the Mississippi and other rivers in our southern States. The explanation ascribing the anomalous course of the river to the deposition of its sediment may be correct to some extent, but we deem it not sufficient to account for so great a difference of level. A glance at the profile of this country will explain the relation between its geological and meteorological condition, which I have before hinted at. Clouds rising from the ocean, borne by aerial currents towards the mountain slope of western California, ascend easily towards the dividing crest. As soon, however, as they pass that line they meet columnar currents of heated air whirling up from the intensely insulated desert flats; dispersion and dissolution of those aqueous deposits of the atmosphere follow naturally, and hence the almost incessant drought the Colorado basin is subject to. Rain destined to fall upon these desert regions needs probably some heavier disturbances in the electro-magnetic action of the air, and hence what is called the rainy season in these regions is nothing more than the hottest time of the year, when electro-magnetism comes to its highest pitch of activity. Equal causes produce similar results, therefore the meteorology of the Colorado basin and Northwestern Sonora are nearly related.. |