Physiographic effects of oscillations.  We have already mentioned several of the most important effects of movements of elevation and depression, such as their effects on rivers, the mantle of waste, and the forms of coasts.  Movements of elevation—­including uplifts by folding and fracture of the crust to be noticed later—­ are the necessary conditions for erosion by whatever agent.  They determine the various agencies which are to be chiefly concerned m the wear of any land,—­whether streams or glaciers, weathering or the wind,—­and the degree of their efficiency.  The lands must be uplifted before they can be eroded, and since they must be eroded before their waste can be deposited, movements of elevation are a prerequisite condition for sedimentation also.  Subsidence is a necessary condition for deposits of great thickness, such as those of the Great Valley of California and the Indo-Gangetic plain (p. 101), the Mississippi delta (p. 109), and the still more important formations of the continental delta in gradually sinking troughs (p. 183).  It is not too much to say that the character and thickness of each formation of the stratified rocks depend primarily on these crustal movements.

Along the Baltic coast of Sweden, bench marks show that the sea is withdrawing from the land at a rate which at the north amounts to between three and four feet per century; Towards the south the rate decreases.  South of Stockholm, until recent years, the sea has gained upon the land, and here in several seaboard towns streets by the shore are still submerged.  The rate of oscillation increases also from the coast inland.  On the other hand, along the German coast of the Baltic the only historic fluctuations of sea level are those which may be accounted for by variations due to changes in rainfall.  In 1730 Celsius explained the changes of level of the Swedish coast as due to a lowering of the Baltic instead of to an elevation of the land.  Are the facts just stated consistent with his theory?

At the little town of Tadousac—­where the Saguenay River empties into the St. Lawrence—­there are terraces of old sea beaches, some almost as fresh as recent railway fills, the highest standing two hundred and thirty feet above the river.  Here the Saguenay is eight hundred and forty feet in depth, and the tide ebbs and flows far up its stream.  Was its channel cut to this depth by the river when the land was at its present height?  What oscillations are here recorded, and to what amount?

A few miles north of Naples, Italy, the ruins of an ancient Roman temple lie by the edge of the sea, on a narrow plain which is overlooked in the rear by an old sea cliff (Fig. 166).  Three marble pillars are still standing.  For eleven feet above their bases these columns are uninjured, for to this height they were protected by an accumulation of volcanic ashes; but from eleven to nineteen feet they are closely pitted with the holes of boring marine mollusks.  From these facts trace the history of the oscillations of the region.