Fossils can be classified according to the rock type associated with the fossils or the mode of preservation. The rock type classification includes siliceous fossils, limestone fossils, dolomite fossils, marl fossils, shale fossils, and sandstone fossils. Fossils in limestone, siltstone and shale are easier to find and collect. For example, trilobites, ammonites, orthoceras, corals and sponge are usually preserved in limestone. Fossils of reptiles and plants are usually found in shale, whereas fossils of dinosaurs, dinosaur eggs and fish are preserved in sandstone. Based on the mode of preservation, fossils can be categorised into body fossils, trace fossils (including relic fossils) and chemical fossils.