On certain coasts long inhabited by civilized man, ancient structures like quays and bridges, which were built in the water, may now be found high above it. Such changes have been noted in the Mediterranean lands, especially in southern Italy and the island of Crete. The so-called Sera-peum at Pozzuoli, near Naples, is a famous and much discussed example of repeated oscillations upward and downward. This structure was built in Roman times and probably began to sink while still in use, as appears from the two ancient pavements, one above the other. Three large monolithic columns of marble, about 40 feet high, are still standing erect, and on each of them is a belt about 10 feet above the ground and 9 feet wide, honeycombed by the boring mollusc, Lithodomus, which still abounds in the neighbouring bay, and many of the shells were actually found in the columns. Evidently, the building was once submerged to a depth of nearly 20 feet, and when under water, the columns were attacked and perforated by the mollusc.

Just when the reelevation began is not definitely known, but there is some documentary evidence to show that it was in progress in the early years of the sixteenth century and was probably completed in 1538, when a volcanic eruption in the neighbourhood resulted in the formation of Monte Nuovo '(see p. 66). For nearly a century past a slow movement of subsidence has been going on.

Fig. 1. - Columns of the "Serapeum "; Pozzuoli, Italy.

Rocks and cliffs long exposed to the action of the surf are worn and marked in a characteristic fashion and cut into terraces, and when found above the level at which the sea can now reach them, are evidences of upheaval at that point. Such well-defined seamarks high above the present sea-level are common in the high latitudes of the northern hemisphere and, in many places, the change is still in progress. The Scandinavian peninsula shows slow changes of level, which constitute an up warp; the south coast of Sweden is stationary or sinking slightly, but elsewhere the movement is upward and increases in amount towards the interior, the successive terraces rising toward the heads of the numerous deep fjords which indent the coast. These facts have been strongly disputed, but have recently been emphatically reaffirmed by the Swedish geologist De Geers, who shows that the isobasic curves, connecting points of equal elevation, form ellipses, the long axis of which coincides with the water-shed between Sweden and Norway. Along this line, the elevation is at a maximum, reaching nearly 1000 feet, and diminishing from the axis toward the periphery.

Such a result can be' explained only by an elevation of the land, not by a withdrawal of the sea, which could not have changed the level of the terraces.

"Raised beaches," filled with the remains of marine animals, are a decisive proof of a rise of the land, or a fall in the sea, and evidence of a similar kind is given by raised coral reefs. Such raised beaches now far above the sea occur in Scandinavia, Great Britain, the West Indies, the west coast of South America, the Red Sea, and elsewhere. The eastern coast of North America shows marks of relatively late elevation, increasing in amount northward. At the mouth of the Connecticut, the highest beach is 40 to 50 feet above sea-level, at Boston it is 75 to 100 feet, on the coast of Maine it is 200, and on that of Labrador 500 feet. On the eastern shore of Hudson's Bay the marine terraces and beaches extend up to 700 feet above sea-level.

Still another kind of evidence of recent elevation may often be gained from the form and character of the coast itself, as will be explained in Part III.

In the geological period (Pleistocene) immediately preceding the recent one, in which we are living, several immense lakes existed in the interior of North America, some around the basins of the present Great Lakes, others in Utah and Nevada. The ancient shore-lines of these vanished lakes may still be seen, for the most part, in admirable preservation; when first formed by the action of the waters, these beaches must have been level, but accurate surveys show that they are no longer so, but have undergone extensive warpings.

Wherever rocks of marine origin occur on land, they prove the elevation of the area where they are found. The great importance of the process is shown by the fact that the larger part of all the continents is composed of rocks which were laid down in the sea and are of all geological dates.