MILWAUKEE COURIERiNORTHERN MEXICO* 'Sketches of travel in Northern Mev.ico during the years, 1345 a»d 1846, written for the New York Sun.The noble fiver separating Mexico from Texas, and which formed the Southern bouudary of ancient Louisiana, is known as the Rio Grande (Grand river), the-Rio Brave del Norte (brave river of the north), the Rio B? act* or the del Norte. Tbe first is becoming the favorite name.The fountains which form this stream take their rise upon the South-west side of the first ridge of the Rocky Mountains, one hundred and fifty miles North of Santa Fe, the Capitol of Mew Mexico. After pur* suing a meandering course South-east, 12 hundred milea from its origin, the Rio Grande gently insinuates itself into tbe Gulf of Mexico, forty miles below Main morns, forming at its mouth a Port of Entry for small vessels drawing six feet and a half of water, and in continuation Steam Boats can immediately carry tbe cargo up ifte river to ftlotumoras. The navigation of the river can still be continued one hundred and fifty miles further, more particularly in the months of.May, June and July, as that, is the time when the snow melts on the Rockv Mountains and causes its great-