Published in 1669, the principles in Steno's 78-page text, "On Solids," are still taught in geology classes today. Science writer Alan Cutler sums up the thesis in his 2003 biography of the scientist, "The Seashell on the Mountaintop,"
The backbone of his system was a simple but tremendously powerful idea. Recognizing that the layers of rock that entombed fossil shells were made by the gradual accumulation of sediment, he realized that each layer embodied a span of time in the past. He saw no way to measure the number of years or centuries involved, and was loathe to speculate, but it was clear that the layers, one on top of the other, formed an unambiguous sequence: The lowest layer had been formed first, the highest last. Depending on their fossils and their sediments, the layers recorded the succession of seas, rivers, lakes, and soils that once covered the land. Geologists call Steno’s insight the "Principle of Superposition." It means that, layer by layer, the history of the world is written in stone.
A decade earlier, James Ussher, an Anglican archbishop in Ireland, published his Biblical chronology of the world. By adding up the reigns of the kings and lifespans of the patriarchs and comparing them with the dates of known historical events, Ussher concluded that the world came into existence on Sunday, October 23, 4004 BC.