Jeckyll, Thomas. Map of the United States and their Territories between the Mississippi River and the Pacific Ocean. Washington DC, 1857 8. 2_8_25 which would eventually lead to Views and Explorations which would eventually lead to significant geopolitical changes in the region These lands were initially shown as small islands on Portuguese charts but they ballooned into a continent sized landmass in later representations Vast swathes of unexplored territory are labeled across the Southwest and Pacific Northwest This impressive map of the United States west of the Mississippi River reflects the latest surveys performed along the newly established Mexican American border by a party led by William H