The Seashell on the Mountaintop: A Story of Science, Sainthood, and the Humble Genius Who Discovered a New History of the Earth by Alan Cutler (2003) is the story of Nicolaus Steno (1638-1686) a figure familiar to every geologist.
Steno was a bona fide genius, a Danish anatomist noted throughout Europe for his dissection skills. His real genius was in the fact that he didn't trust what "experts" said or had written about something, he investigated everything himself. He was invited to join the Accademia del Cimento (Academy of Experiment) in Florence, Italy where he spent a number of years working with other Italian scientists.
While there, he had the opportunity to dissect the head of a huge shark caught by fishermen in nearby Livorno on the coast and noticed that the shark's teeth looked exactly like glossopetrae or "tongue stones" found preserved in rocks. To us today, it's an obvious conclusion that glossopetrae were fossil shark teeth but, in the 17th century, the concept that fossils are the remains of once-living organisms was highly controversial. Many still believed the ancient claims that fossils were objects which "grew" in the rocks and any resemblance to once-living things was simply coincidence.
Steno's new-found interest in fossils led him to study sedimentary rocks in the surrounding countryside which resulted in the writing of De solido intra solidum naturaliter contento dissertationis prodromus or Preliminary discourse to a dissertation on a solid body naturally contained within a solid in 1669. He developed a number of concepts as a result of this study which sound quite simple but actually have profound consequences by allowing us to work out the relative ages of rock strata. These principles are:
1. Original horizontality - the idea that sedimentary strata are originally deposited in horizontal layers. If they are no longer horizontal, something later has disturbed them (e.g. folding during mountain building).
2. Superposition - the idea that in a sequence of layered sedimentary rocks, the oldest rocks are found near the bottom of the sequence and the youngest near the top.
3. Lateral continuity - the idea that sedimentary rocks extend laterally as a unit. This allows us, for example, to match up rock units from one side of a valley to the other by assuming they were once laterally continuous.
4. Cross-cutting relationships - the idea that if a rock unit is cut by something (e.g. an igneous intrusion, a faulty, etc.), the structure cutting through is younger than the object doing the cutting.