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a hollow, with a black base and grey shelv-
ing sides. This was generally accepted, but
later investigations have contradicted its uni-
versality. Regarding the cause of these hol-
lows, Wilson said:–
Whether their first production and sub-
sequent numberless changes depend upon
the eructation of elastic vapours from be-
low, or upon eddies or whirlpools commenc-
445