H. P. Lovecraft began to write The Shadow Over Innsmouth in 1931. In one part of Zadok Allen’s history, he mentions “certain signs” belonging to the ‘Old Ones’ which have powers over (or against) the Deep Ones:
“…sarten signs sech as was used onct by the lost Old Ones … them old magic signs as the sea-things says was the only things they was afeard of … In some places they was little stones strewed abaout—like charms—with somethin’ on ’em like what ye call a swastika naowadays. Prob’ly them was the Old Ones’ signs…”
While this use of the swastika may seem incongruous to a late 20th / early 21st century audience, it is in fact a very old symbol, appearing in ancient North America, Europe, Africa and India. What most likely prompted the use of it here (in the Pacific) in HPL’s story were the public speculations of James Churchward from 1926 on:
We know HPL was familiar with Churchward’s theories because he mentions him by name in Through the Gates of the Silver Key (1932) and Out of the Aeons (1933). Reading the first 3 paragraphs of the article above gives us a clear view at the then popular ‘lost continent’ stories in the public eye at the time, and the ‘evidence’ used to support them.