DURING the first years of the present century, the tide alike of emigration and exploration had set toward the northern territories of the United States. The discovery of the sources of the Mississippi and of those of its chief affluents, however, left little more to be done in that direction, and, in 1838, a new chapter in the history of modern American geography was ushered in by the sending out of a naval exploring expedition, under Captain Wilkes, with orders to make surveys along the western coast, especially of the bay now known as that of San Francisco. On the arrival of Wilkes at Yerba Buena, as what has since become the gate of the Pacific was then called, the town consisted merely of a large frame building occupied by an agent of the Hudson’s Bay Company, a store kept by an American, a billiard room and bar, a poop cabin of a ship used as a residence by a captain, a blacksmith’s shop, and a few sheds, all of which had sprung up at intervals since California, deserted, as related above, by its early colonists, had become the prey of trappers, miners, and other adventurers of various nationalities. The most prominent man then in California was a Swiss named Sutter, who had bought of the Mexican Government a tract of land thirty leagues square, and was now employed in laying it out under the imposing name of New Helvetia. He had already built a fort near the junction of the Sacramento and American rivers to which he had given his own name, little dreaming that the discovery of gold close at hand would shortly make that name world-famous, as the most powerful emigration magnet of the day. Equally unconscious of the approaching, almost miraculous change in the aspect of affairs on the south-western coast of North America, Wilkes completed his survey, and returned home to report the harbor of Yerba Buena to be one of the finest, if not the very best in the world; and, a little later, the Government sent out an expedition, under Lieutenant John C. Fremont, to explore the southern half of the Rocky Mountain Range, with a view to the discovery of a good route to California. Illustration: JOHN C. FREMONT. Fremont left the mouth of the Kansas, at the head of a party of about twenty, about the 2d of May, and followed its course across the state of the same name, till he reached the barren banks of the Platte, or Nebraska. Here he decided to follow the southern fork of that important tributary of the Mississippi, and a march of a few miles brought the party into the districts occupied by the Arapaho and Cheyenne Indians, the former of whom at first appeared disposed to dispute the passage of the white men.