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Jesse L. Weston wrote:
From Ritual to Romance p. 48.We may also note the fact that the Grail castle is always situated in the close vicinity of water, either on or near the sea, or on the banks of an important river. In two cases the final home of the Grail is in a monastery situated upon an island. The presence of water, either sea, or river, is an important feature in the Adonis cult, the effigy of the dead god being, not buried in the earth, but thrown into the water.