98 GEOLOGY. 



also a visitor of the Sandwich Islands, having witnessed there the outbreak of the notorious 

 " Kuaroa," knew instantly what it was, and coolly remarked, touching bottom with a sounding- 

 pole : "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 phenom- 

 enon, 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 lupture with an average width of about thirty feet. The edges of this moun- 

 tain 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 hereto- 

 fore 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 depo- 

 sition 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 sufiicient 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 j^ass 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. 



Eain 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. 



