The lowest strata in the Isle of Wight are the coloured marls and blue-grey shales we have already observed in Sandown Bay, which run through the Island to Brook Bay. They are known as the Wealden Strata, because the same strata cover the part of Kent and Sussex called the Weald. They consist of marls and shales with bands of sandstone and limestone. The marls and shales in wet weather become very soft, and flow out on to the shore, causing large slips of land.[1] Now, what we want to find out is what the world was like ages ago, when these Wealden Strata were being formed. We have learnt something of how clays and sandstones and limestones are formed: to learn more we must see what sort of fossils we can find in these rocks. "Fossil" means something dug up; and the word is generally used for remains of animals or plants which we find buried in the rocks. We have seen shells in these strata. These we must examine more closely. And as we walk on the shore we shall find other fossils. In the marls and shales exposed on the shore we are pretty sure to see pieces of wood, black as coal, sometimes quite large logs, often partly covered with shining iron pyrites. Perhaps you say--I hope you do--there must have been land not far away when these marls and shales were forming. Always try to see what the things we find have to tell us. The sort of place where we should be most likely to find wood floating in the sea to-day would be near the mouth of a great river like the Mississippi or the Amazon,--rivers which bring down numerous logs of wood from the forest country through which they flow. Examine the shales and limestone bands. On the surface of some of the paper-shales are numbers of small round or oval white spots. They are the remains of shells of a very minute crustacean, Cypris and Cypridea, from which the shales are known as Cyprid shales. In other bands of shale are quantities of a bivalve shell called _Cyrena_. There is a band of limestone made up of Cyrena shells, containing also little roundish spiral shells called _Paludina_.[2] This limestone resembles that called Sussex or Petworth Marble, which is mainly composed of shells of Paludina, but some layers also contain bivalve shells. It is hard enough to take a good polish, and may be seen, like the similar Purbeck marble, in some of our grand old churches. Another band of limestone running through the shales is made up of small oysters (_Ostrea distorta_).