On the 15th of April the members of the corps, instruments, camp utensils, and travelling gear, were safely stowed on board the little steamer _Tortuga_,—a name that implied slow progress, which to our satisfaction her speed decried. At eleven A. M., having bid farewell to our newly acquired friends, we left the anchorage with their “Good speed,” and after threading her way among the vessels in the roadstead the little steamer puffed her way up the broad expanse of the La Plata estuary. The balance of the day was occupied in arranging instruments for river observations, the while listening to praises lavished by the captain upon the “worthy” qualities of his little propeller, of which he was the architect and builder. During the evening he regaled us with incidents of his life in California and the East Indies. His adventures in California received occasional illustrations from a genial individual introduced as Padre Simon, the prefix having been conferred—as we afterwards learned—from his zealous support of the Catholic dogmas, theoretically. As the padre was eventually enlisted in our corps, we will foreshadow some of his peculiar characteristics. In form he was of medium height, with a rotund outline visibly inclining to jovial obesity; his face was in-dyed with a complexion blending with the Roman auburn of his hair, which gave a warm glow to his expression when lighted with a smile. In the first generation of descent from Irish parentage, he retained the full impression of inconsistency in the practical adaptation of his habits to the faithful index of goodness ingrafted from the maternal stock. Guileless in thought, when free from temptation, he possessed a ready facility of excusing his habits of excess with the plea of saving grace administered under the seal of confession. With this hint, in forecast of development, we will proceed in the relation of events transpiring during the river voyage. On the morning of the 21st of May, after having been subjected to our full share of vexatious delays, incident to the provincial _poco pocoism_ of the guarda and custom-house officials, the steamer gained the river post of Santa Anna on the Pilcomayo, two miles above its mouth. At Santa Anna they found the well-known American naturalist, Diego Dow, waiting for an opportunity to obtain sufficient aid to attempt the exploration of the Pilcomayo as far as the reputed settlement of Tenedos, which rumor located on a confluent stream rising and flowing eastward through the valleys of the Andean spur that reached into central La Plata. The ultra-savage disposition of the wandering tribes on the banks of these rivers, having defeated every previous attempt made to establish trading-posts, but few had been found willing to incur the hazard proposed by Mr. Dow. Even the indomitable Jesuits had been foiled in all their endeavors to conciliate the Indians in degree sufficient for the establishment of missions preliminary to their subjugation.