''You are about to read a lot about dirt, which no one knows very much about.'' So begins the book that brings mystery and magic to ''that stuff that won't come off your collar''. Thomas Jefferson, Saint Phocas, Darwin and Virgil parade through this thought-provoking work, taking their place next to the dung beetle, the compost heap, dowsing and the microscopic biota that till the soil. Whether Logan is traversing the far reaches of the cosmos or ploughing through our planet's crust, his delightful, elegant and surprisingly soulful meditations greatly enrich our concept of ''dirt'', that substance from which we all arise and to which we all must return.