The Lewis and Clark Expedition (1803-1806), headed by Meriwether Lewis and William Clark, was the first American overland expedition to the Pacific coast and back. It was only the third recorded transcontinental crossing of North America north of Mexico by a person not of the indigenous peoples of the Americas, having been preceded to the Pacific coast (on July 20, 1793) by a Canadian expedition led by explorer Sir Alexander Mackenzie. The American expedition to the Pacific northwest was intended to study the Indian tribes, botany, geology, Western terrain and wildlife in the region, as well as evaluate the potential interference of British and French Canadian hunters and trappers who were already well established in the area. President Thomas Jefferson selected Captain Meriwether Lewis to lead the expedition, afterwards known as the Corps of Discovery. Lewis selected William Clark as his partner. Because of bureaucratic delays in the U. S. Army, Clark officially only held the rank of Second Lieutenant at the time, but Lewis concealed this from the men and shared the leadership of the expedition, always referring to Clark as "Captain".