Then Laura heard a strange, low, distant sound.
“What’s that!” she said.
Pa listened.
“Cattle, by George!” he said. "Must be the cattle herds going north to Fort Dodge."
***
Is the expression "by George" (1) some kind of special idiom meaning something other than some one named George? For example, meaning "Judging from the sound".
Or, (2) it just plainly means that the cattle is being herded by someone named George?
(edited to resolve formatting problems)