From the observation of fossilized shells embedded in sedimentary rocks on mountains high above sea level, de Maillet recognized the true nature of fossils. Not appreciating that the land might rise, he concluded that the earth had originally been entirely covered by water (a theory of René Descartes), which had since been steadily lost into the "vortex". De Maillet derived a rate of fall of sea level, of about three inches per century, from sites where former ports were now above sea level. Working back to the highest mountains, he reached a figure of 2.4 billion years for the age of the earth, so he thought it quite reasonable to accept that at least 2 billion years had passed since the earth was covered with water. Of course, this attempt is flawed in several ways, but it contains the seed of real progress. He saw the importance of slow natural processes operating over long periods of time, forming and shaping the earth. He introduced the idea that the Earth might be billions of years old, which even a century and a half after his death would have been denied.