I’m reading Philip Pullman’s prequel to The Golden Compass series and dust is rather important in that book (it is called The Book of Dust) and rather a lot more than earth. And probably actual dust is also a lot more than earth. Probably dust is just a way to say a very small amount of something that makes it hard to identify as anything else. Like, some dust is ash. Some is skin. Some is wood. Some is lint. And perhaps that is all earth is, too – just a large collection of disparate objects and substances.