Final answer:
The 'free-flowing substance' that Steno referred to is likely to be water or other fluid materials, which can transport and deposit sediments. This is important for the formation of fossils like stromatolites, which are formed by cyanobacteria in such sediment-laden, water-based environments.
Step-by-step explanation:
Based on what you've asked about the 'free-flowing substance' Nicolas Steno referred to, it's likely that this term is an allusion to water or other types of fluid material in the Earth's crust. The concept relates to the formation of fossils, which often takes place in sedimentary layers that were originally free-flowing substances, such as silt or mud. Over time, these layers harden, entombing the biological material and forming fossils. The concept is central to Steno's law of superposition, which states that the oldest layers are at the bottom and the youngest are at the top, thus reflecting a timeline of Earth's life.
For example, consider the formation of stromatolites, some of the oldest fossils on Earth. These structures are formed by cyanobacteria that create layers of sediment in shallow, water-filled environments. Over time, the layers of sediment hardened into rock, preserving the structures created by the cyanobacteria and providing a snapshot of life from over 3 billion years ago.
Further examples can be found in sedimentary rock layers that show signs of a previous aqueous environment. These include not just stromatolites, but also other types of sediment that contain deposited minerals. Such formations offer important clues about the past existence of water-based environments and the life forms they harbored, all thanks to the 'free-flowing substance' that Steno discussed.
Learn more about Fossil Formation