The victim was a seamstress, found dead in a bean patch, strangled by her own scarf. The suspect was a local creep who insisted he had nothing to do with the crime and was far away when it occurred. How did one detective prove what really happened? With dirt.
At the turn of the last century, no one was particularly worried about leaving evidence at crime scenes, which is why German detectives examining the area near the strangled body of Eva Disch found the murderer's mucus-filled handkerchief right next to her. Unless the handkerchief was monogrammed, it couldn't point to a suspect -- that is, until George Popp came on the scene.
Popp examined the handkerchief and found that the mucus in it was full of snuff, coal dust, and hornblende. Hornblende is a component of many types of rock, including granite. A look at the nearby residents turned up one man, Karl Laubach, who worked at both a gasworks where coal was burned and a gravel pit where hornblende was among the materials. When they hauled Karl in to be questioned, detectives checked under his nails and got all the components on the handkerchief -- except, thank goodness, the mucus.
What they didn't get was a confession. Laubach insisted he'd been nowhere near the scene of the crime. Unfortunately for him, he didn't hem his trousers. Popp took a look at the bottom edges of his pant legs and found layers of dirt. At first the dirt didn't look anything like the stuff found in the bean field. It had a layer of finely crushed mica that wasn't present in the field. Only under that layer could Popp find all the minerals he'd found in a sample of dirt from the field.
What was the outer layer of dirt? Popp scouted around, taking samples, and found that the road between the field and Laubach's house had that particular type of crushed mica on it. Not only did geology help Popp prove that Laubach killed Disch, it showed him the route Laubach took as he walked home afterwards.