in the call of the wild , what exploit did Buck accomplish that made John Thornton jubilant ?
I believe you are referring to the bet Buck won for John in Chapter Six. In Chapter Seven, John is able to pay off all of his debts with the winnings and go in search of the fabled gold mine.
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.
Call of the Wild
