For what place are Thornton and his friend searching? What do they hope to find there?

Thornton and his partners are traveling East in hopes of finding a fabled lost mine filled with gold.

When Buck earned sixteen hundred dollars in five minutes for John Thornton, he made it possible for his master to pay off certain debts and to journey with his partners into the East after a fabled lost mine, the history of which was as old as the history of the country.

Dying men had sworn to it, and to the mine the site of which it marked, clinching their testimony with nuggets that were unlike any known grade of gold in the Northland.