 His observations led him to formulate important stratigraphic concepts (i.e., the "law of superposition" and the "principle of original horizontality").In the 1790s, William Smith hypothesized that if two layers of rock at widely differing locations contained similar fossils, then it was very plausible that the layers were the same age.William Smith's nephew and student, John Phillips, later calculated by such means that Earth was about 96 million years old.