Rosas immediately fled on board a British vessel in the harbor of Buenos Ayres, with the intention of proceeding to England at the earliest opportunity. He had been engaged for some weeks in securing large amounts of treasure, in apparent preparation for such a flight. General Urquiza immediately followed up his victory by investing the city of Buenos Ayres. Deprived of its governor, of course it could make no long defense, and steps had already been taken to organize a constitutional government under the new auspices. The intelligence of the fall of Rosas had created the liveliest satisfaction in England, and was followed by an immediate and very considerable rise in the market value of Buenos Ayres bonds. This change in the political prospects of that portion of South America, it is believed, will lead to a largely increased emigration thither from the southern parts of Europe. The government of Rosas has been for many years an object of terror and distrust in Buenos Ayres, and has greatly retarded the industry and progress of the country. It has at last been overthrown--not by the intervention of foreign states, but by the independent exertions of the people themselves. General Urquiza, the successful soldier, seems disposed to use his power so as to promote the best interests of the country, and under his guidance a new organization of the several states may be expected.----The Congress of _Venezuela_ was still in session on the 10th of March. The affairs of the country were highly prosperous.----The revolted convicts at the Straits of Magellan had been seized by the British war steamer Virago, and taken heavily ironed to Valparaiso. There were in all 350, of whom 180 were taken from the British brig Eliza Cornish, which they had seized:--the rest had taken the American bark Florida, but were afterward subdued by a counter-plot on board, and were delivered up. The officers of the Cornish had been shot in cold blood by the miscreants, who were guilty of shocking barbarities. They were landed at Valparaiso, February 25, and delivered over to the authorities.----In _Peru_ an expedition had been organized by General Flores, against Ecuador. It is said he has enlisted two or three thousand men, and sent out four or five vessels loaded with men and munitions, for an attack on the city of Guayaquil. Great excitement prevailed at the latter place, where preparations had been made to give the invaders a warm reception.----Panama papers record the successful result of an expedition to the reputed gold placers on the coast of Choco, in the southern part of the kingdom of _New Grenada_, about 150 miles south of Panama. About 1500 ounces of pure gold dust were exhibited in the latter city, as the first-fruits of the enterprise. There seems to be no doubt of the existence of the _oro_ in that vicinity in large quantities. GREAT BRITAIN.