This all relates to US geography & history.
1) Please look at a map of the US. Divide it roughly into 3 equal sections going from left to right. The right most section is the Eastern third, basically from Chicago going east to the coast.
2) Vanishing frontier refers to the boundless western US. After the Louisiana Purchase, American were encouraged to move west and settle the land. The land seemed endless, untouched, and lawless. It was referred to as the frontier. However as more American moved west, settled down, built homes & roads, established laws and customs, the frontier started to disappear, hence "the vanishing frontier"