Yes, I'm still reading Minnesota geology, but but I received a complimentary copy, from Adventure Publications, of a stunning paperback titled The Storied Agate: 100 Unique Lake Superior Agates by James Magnuson.  So naturally I took a side trip.  It's not a book of science but the intro gave me a good clue as to how to explain the formation of agates.  It is page after page of Minnesota agates, each more beautiful than the next, interspersed with serene anecdotes and descriptions.  I would recommend this book.

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