_Monday, May 20._--A guard was kept up all last night, but every thing was quiet, and as I said yesterday, I believe the danger, if any was to be apprehended, had passed away. But the excitable feelings of the hundreds of Americans now under arms had to be indulged, and hearing that a camp, mostly composed of foreigners, situated about seven miles from Sonora, had mounted Mexican, Chilian, and French flags, (what truth there is in the report, I know not,) they have started out this morning to avenge the insult, and chastise the temerity of the "greasers" and "outsiders." I sincerely trust there will be good sense enough in the party to refrain from wanton aggression. Gov. Burnett has sent Hon. John Bidwell and Judge H. A. Schoolcraft in charge of the block of stone contributed by California to the Washington Monument. It is thus described: "This block of gold-bearing quartz, is from the Mariposa diggings, near Fremont's mines, and weighs about one hundred and twenty-five pounds. In shape it is irregular, approaching a square, its sides varying from eighteen to twenty inches in length. It averages in thickness nine inches--across its face diagonally it is twenty-one inches by measurement. Very little gold is perceptible to the naked eye, but it is estimated to contain about eighty dollars worth." Since the above events were recorded, another most disastrous fire has occurred in the city of San Francisco. It broke out in the Sacramento House, situated in the wealthiest portion of the city, on the 17th of June, 1850, at eight o'clock, A.M., and in the short space of three hours, about two-thirds of the wealthiest district was consumed. The shipping in the harbor was only saved by the greatest exertion. The entire loss is estimated at from three to four millions of dollars. This event occurring so soon after the previous fire, has had a depressing and gloomy effect upon the business operations, not only of San Francisco, but of Sacramento City also. Many of the heaviest trading houses have been entirely ruined; and others brought to the verge of it. Several individuals, including the Mayor of the city, distinguished themselves by their noble and generous exertions to arrest the progress of the fire and save property. The emigration to California by way of the overland route is six times as great during the present year as it was in 1849. The last company left Council Bluffs, on the 15th of June. They brought up the rear of near four thousand wagons, ten or twelve thousand persons, and about twenty thousand head of horses and cattle. The continued success of the gold diggers and the extraordinary prospect in regard to the quantity that will be obtained during the mining season of 1850, serves to keep up the excitement and to allure the emigrant to the golden land.