Respecting the eastern or Littorine provinces of the Republic, as I have ventured to call them, the most valuable data existing are, first, those collected by the Jesuits, and next, the various reports and memoirs drawn up by the officers employed to fix the boundaries under the treaties between Spain and Portugal of 1750 and 1777. The especial qualifications of the individuals, particularly of those employed in the last case, the length of time spent upon the service (more than twenty years), and the enormous expenses incurred by Spain in the endeavour to complete that survey, led to a large accumulation of invaluable geographical data respecting extensive ranges of country never before properly examined, much less described. Nor were the labours of the officers in question confined to the frontiers. They fixed, as I have stated in Chapter VIII., all the principal points in the province of Buenos Ayres, made surveys of the great rivers Paranã and Uruguay, and of their most important tributaries; and drew up many notices of great interest respecting the countries bordering upon the higher parts of the Paraguay, which the pretensions of the Portuguese in that direction rendered it requisite for them to explore with more than ordinary care and attention[2]. M. Walckenaer's publication at Paris, in 1809, of the Travels of Don Felix Azara, one of the King of Spain's commissioners on that service, contains a general review of the labours of those officers, and is perhaps the best work in print upon the countries which it describes; still it can only be regarded as a very imperfect sketch of the information collected by one of many able men employed upon that particular service. Another of the commissioners, Don Diego Alvear, drew up an historical and geographical work upon the provinces of Paraguay and the Missions, quite equal in interest, if not more so, than that by Azara, for a MS. copy of which I have to thank his son, the present General Alvear.