Because of the location on the Bosporus Strait, the Byzanite Empire:

a historian researching the immigration policies of the early 1900s would expect a source written today,rather than in 1910 to :

( a ) be more interested in objective data and statistics.

a big wall across the mexican border so no drugs, terrorist, or illegal immigrants can come through : )    

the answer to your question is a. the salt plains

in 1811, he led an expedition, known as the george c. sibley expedition, to improve relations with the pawnee and kansa tribes, and also to locate the rumored jefferson's salt mountain