The ocean could not, indeed, like the land have risen or sunk only in parts. Except where it is forcibly held at a higher level by some strong force of local attraction, this would be impossible. Yet causes might exist which should affect the whole ocean-surface. One such may be mentioned. The rising or sinking of great tracts of land could not fail to affect the ocean-level. That it must do so may be easily understood. Let us fill a basin with water, to within an inch of the brim, and then slowly lower into the water a ball, or any other solid body. As the ball enters, the level of the water rises--_must_ rise. Then let us slowly lift the ball out again; and as it leaves the water, the level of the latter sinks--_must_ sink. Precisely the same result would follow upon the rising or sinking of some great continental mass. When it rises, the ocean contains less land, and the water-level must go down. When it sinks, the ocean contains more land, and the water-level must go up. This question has been long under discussion. For many years, as with other questions, each side looked upon its own view for the time being as the only right one. But--as again with many difficult problems--the fairer and wider view recognises the possibility of truth in both explanations, and probably in the end both will prove to be in a measure right. Many other kinds of rock, as well as sandstone, have been built under ocean-waters in past ages, constructed of minute particles, piled together, and hardened. Some sorts, like sandstone, are only or chiefly mineral; while others are only or chiefly “organic”--that is, are mainly composed of the remains of “organised” or living beings. About the latter much has to be said in future chapters. A very few words in the close of this chapter, as to the classifying of different kinds of Rocks, may not be amiss. Ocean-deposits, or Ocean-buildings, are often roughly divided into three groups. In “Deep-Sea Deposits” are included all that lie beyond the Hundred-fathom Line, or a depth of six hundred feet. In “Shallow-water Deposits” are included all that lie between six hundred feet deep and low-water mark. In “Littoral Deposits”--the word coming from the Latin for “shore”--are included all deposits that lie on shore-lines, between the highest and the lowest tides. At first sight the tide-deposits might seem to be but a small matter. Not so small, really! Earth’s coast-lines reach to something like one hundred and twenty-five thousand miles in length. Since the medium breadth of ground affected by tides is about half-a-mile, the whole tidal area amounts to over sixty thousand square miles. No mean workshop this, under the auspices of crushing and grinding billows. On shore-lines building takes place rapidly.