Ages must have passed while the ocean flowed over this part of the world, and the chalk mud, with its varied remains of living things, gradually accumulated at the bottom. At last a change came. Slowly the sea bed rose, till the chalk, now hardened by pressure, was raised into land above the sea level. As soon as this happened, sea waves and rain and rivers began to cut it down. There is evidence here of a wide gap in the succession of the strata. Higher chalk strata, which probably once existed, have been washed away, while the underlying strata have been planed off to an even surface more or less oblique to the bedding-planes. The highest zone of the chalk in the Island (that of _Belemnitella macronata_) varies greatly in thickness, from 150 ft. at the eastern end of the Island to 475 at the western. The latest investigations give reason to conclude that this is due to gentle synclines and anticlines, which have been planed smooth by the erosion which preceded the deposition of the next strata,--the Eocene.[11] At Alum Bay the eroded surface of the chalk may be seen with rolled flints lying upon it, and rounded hollows or pot-holes, the appearance being that of a foreshore worn in a horizontal ledge of rock, much like the Horse Ledge at Shanklin. The land sank again, but not to anything like the depth of the great Chalk Sea. We now come to an era called the Tertiary. The whole geological history is divided into four great eras. The first is the Eozoic, or the age of the Archaean,--often called Pre-Cambrian--rocks; rocks largely volcanic, or greatly altered since their formation, showing only obscure traces of the life which no doubt existed. Then follow the Primary era, or, as it is generally called, the Palaeozoic; the Secondary or Mesozoic; and the Tertiary or Kainozoic. Palaeozoic is used rather than Primary, as this word is ambiguous, being also used for the crystalline rocks first formed by the solidification of the molten surface of the earth. But Secondary and Tertiary are still in constant use. These long ages, or eras, were of very unequal duration; yet they mark such changes in the life of animal and plant upon the earth that they form natural divisions. The Palaeozoic was an immense period during which life abounded in the seas,--numberless species of mollusca, crustaceans, corals, fish are found,--and there were great forests, which have formed the coal measures, on land,--forests of strange primeval vegetation, but in which beautiful ferns, large and small, flourished in great numbers. The Secondary Era may be called the age of reptiles. To this era all the rocks we have so far studied belong. Now we come to the last era, the Tertiary, the age of the mammals. Instead of reptiles on land, in sea and air, we find a complete change. The earth is occupied by the mammalia; the air belongs to the birds such as we see to-day. The strange birds of the Oolitic and Cretaceous have passed away. Birds have taken their modern form.