Describe the Principle of Lateral Continuity
From Tuscany to the World
Less than two years after dissecting the head of the great white shark, Steno completed a manuscript on the geology of Tuscany. The locale was a shrewd choice; it celebrated the homeland of his patrons, the Medici. Steno intended this 78-page volume, De solido, simply as an introduction to a more in-depth dissertation, a dissertation he never published. To chart a course for geology, however, De solido was enough.
Today, geologists and paleontologists typically document the rock outcrops where they work through stratigraphic sections, diagrams of rock layers shown in cross-section. This hadn't been done before Steno's time, and though the diagram he made was more abstract and based on more assumptions than is common today, he pioneered the practice. More importantly, he outlined principles that underlie geologic research.
Three basic kinds of rock cover the globe. Igneous rocks result from melting rock, such as lava flows. Metamorphic rocks show dramatic change due to extreme heat or pressure. Sedimentary rocks result from sediments eroded by wind or water that have since been redeposited. Paleontologists prize sedimentary rocks because they bear fossils, and paleontologists rely on "Steno's Principles" to organize fossils in time and space.
The first and most important of Steno's principles seems laughably apparent today, but it was far from obvious at the time. Known as the "principle of superposition," it states that the sediment layers are deposited in sequence, with the oldest layers on the bottom and newest layers on top. "When the lowest [rock] stratum was being formed," he wrote, "none of the upper strata existed." Long before anyone could determine absolute ages of rock layers, geologists could, by relying on Steno's first principle, at least figure out which rock layers were older.
When watching experiments in the alchemy workshop of his mentor Ole Borch, Steno had observed how sediment settles to the bottom of a container. His youthful observations served him well in geology. Steno knew that when water or sediment fills a basin, its upper surface levels out to be smooth and parallel to the horizon, even if it was deposited onto an irregular surface. Succeeding layers of sediment are deposited in the same fashion. This has become known as Steno's "principle of original horizontality," and it has helped geologists understand that layers of sediment lying at an angle to the horizon were tilted or folded after they solidified.
Steno's final principle is the "principle of lateral continuity," which says that sediment layers spread out until they reach an obstacle that keeps them from spreading further, the way soup spreads out in a bowl until it reaches the sides of the dish. "Wherever bared edges of strata are seen," he wrote, "either a continuation of that same strata must be looked for or another solid substance must be found that kept the material of the strata from being dispersed."
Modern geologists diagram stratigraphic columns using the general principles developed by Steno. (Image by Michon Scott, Earth Observatory)
Relying on these principles, Steno tried to recount the geologic history of Tuscany. He produced a six-phase history. In the first phase, the region was flooded and layers of sediment deposited. In the second phase, the area drained. In the third phase, the rocks collapsed to form mountains and valleys. Steno envisioned this crustal collapse resulting from the rocks giving way as air pressure or water carved out caverns underground, similar to how water washes out pockets of earth under highways to create potholes. Steno believed this cycle of flooding, draining, and collapsing happened twice, and he related it to Scripture. The first flood could have been when "the spirit of God moved on the face of the waters." The second flood might have been Noah's Flood, though Steno didn't say for sure.
He was wrong about the vast underground caverns leading to crustal collapse, but modern geologists understand that the movement of crustal plates certainly can fracture bedrock and build mountains. In De solido, Steno ambitiously claimed that the geology he uncovered in Tuscany held true for the entire globe. He was wrong about that too, but correct in believing that studies of rock strata could eventually lead to a global timescale.
De solido needed approval from Florentine censors before it could be published, but luckily for Steno, the censors were good friends who raised no objections. By the time the manuscript rolled off the press, however, he had reluctantly left Florence.
The geologic upheavals Steno described in his manuscript might have hinted at similar upheavals in his personal life because he made a decision that would surely disappoint his Lutheran friends back home. He converted to Catholicism. In the midst of his conversion, he received an ironic summons from King Frederick III to return to Denmark. The kind of well-paid post in Copenhagen that had eluded him years before was suddenly available, but the now-Catholic Steno didn't want to go. He left Florence, but spent the next 20 months on a journey of nearly 4,000 miles, not exactly a direct route to the Danish court. Steno had reached Amsterdam when he received word that Frederick III was dead, and he was off the hook.
Historians don't know what Steno planned to do next. On the long road to Denmark, he had seen some of Europe's best geological wonders, including the Alps and Mount Vesuvius, and the next logical step might have been to follow up De solido with a more in-depth work, but no manuscript has been found. Years later, after Steno died, Gottfried Leibniz—best remembered for his dispute with Isaac Newton over the invention of the calculus—was so convinced that Steno must have kept working that he searched diligently for any writings Steno might have left behind. Unfortunately, if any manuscript existed, it wasn't preserved. In the last phase of his life, the restless Steno changed course once again.
next: Legacy
back: The Nature of Fossils
Steno envisioned a six-stage history for the geology of Tuscany, including cycles of flooding, draining, and crustal collapse. (Image from The Meaning of Fossils by Martin J.S. Rudwick)
Source: https://www.earthobservatory.nasa.gov/features/Steno/steno5.php
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