The 1840 edition introduced the definite article the in front of earth, but this is clearly a typo, despite it being copied into the early RLDS textual tradition. It was removed from the RLDS text in 1908. To be sure, the expression “the banks of the earth” is quite unexpected for speakers of English. There are two other instances in the text of “banks of earth”, one of which occurs earlier here in Alma 49:22:
As expected, there are no instances of “banks of the earth” in the original text.
Summary: Maintain the original two occurrences of the phrase “banks of earth” in Alma 49:22; the intrusive the that entered the 1840 edition for the second occurrence is a typo.