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I’ve never seen the construction before, except in Spanish for dialogue and Toni Morrison using the variation “;—” in Beloved for denoting a strong pause (well, that’s what we decided in my English class).
How does Henry Adams use “:—”? Is this grammatically correct? How can I incorporate this into my own sentences?
Among senses, smell was the strongest:—the smell of hot pine-woods and sweet-fern in the scorching summer noon; of new-mown hay; of ploughed earth
— Henry Adams, in The Education of Henry Adams