Although a line of separation, formed by the basin of the Rio Branco, is now established throughout the whole extent of the country, to which we are desirous of directing the inquiring zeal of travellers, it must yet be admitted, that our geographical knowledge of the district west of this valley between 62° and 66° long., has made no advance whatever for at least a century. The repeated attempts made by the Government of Spanish Guiana since the expeditions of Iturria and Solano, to reach and to pass over the Pacaraima Mountains, have been attended by very unimportant results. When the Spaniards, in proceeding to the missions of the Catalonian capuchins of Barceloneta, at the confluence of the Caroni and the Rio Paragua, ascended the last-named river southward to its junction with the Paraguamusi, they founded at this point the mission of Guirion, which, at first, bore the pompous appellation of Ciudad de Guirion. I place it in about 4° 30′ north latitude. From thence the Governor Centurion, in consequence of the exaggerated accounts given by two Indian chiefs, Paranacare and Arimuicapi, respecting the powerful tribe of the Ipurucotos, was excited to search for ‘El Dorado,’ and in carrying what were then called spiritual conquests still further, founded, beyond the Pacaraima Mountains, the two villages of Santa Rosa and San Bautista de Caudacacla. The former was situate on the upper eastern bank of the Uraricapara, a tributary of the Uraricuera, which I find in the journal of Rodriguez under the name of the Rio Curaricara; the latter, at from 24 to 28 miles further east-south-east. The astronomo-geographer of the Portuguese Boundary Commission, Captain Don Antonio Pires de Sylva Pontes Leme, and the Captain of Engineers, Don Ricardo Franco d’Almeida de Serra, who between 1787 and 1804, surveyed with the greatest care the whole course of the Rio Branco and its upper tributaries, call the most western part of the Uraricapara, “The Valley of Inundation.” They place the Spanish mission of Santa Rosa in 3° 46′ north lat., and mark the route that leads from thence northward across the mountain chain to the Caño Anocapra, a branch of the Paraguamusi, which forms a connecting passage between the basin of the Rio Branco and that of the Caroni. Two maps of these Portuguese officers, embracing all the details of the trigonometrical survey of the bends of the Rio Branco, the Uraricuera, the Tacutu, and the Mahu, were most kindly communicated to Colonel Lapie and myself by the Count of Linhares. These valuable unpublished documents, of which I have availed myself, are still in the hands of the learned geographer, who long since began to have them engraved at his own expense. The Portuguese sometimes call the whole of the Rio Branco by the name of Rio Parime, and sometimes limit this appellation to one branch only, the Uraricuera, somewhat below the Caño Mayari and above the old mission of San Antonio. As the words _Paragua_ and _Parime_ alike imply water,