The most important point to be gained here is that every line of stratification and every change in the character of the sediments is due to some change of corresponding magnitude in the conditions under which the rock was formed. The slight and local changes in the conditions occur frequently and mark off the individual laminæ and strata, while the more important and wide-spread changes determine the boundaries of the groups of strata and the formations. Strata are subject to constant lateral changes in texture and composition, _i.e._, a bed or formation rarely holds the same lithological characteristics over an extended area. There are some striking exceptions, especially among the finer-grained rocks, like slate, limestone, and coal, which have been deposited under uniform conditions over wide areas. It is the general rule, however, particularly with the coarse-grained rocks, which have been deposited in shallow water near the land, that the same continuous stratum undergoes great changes in thickness and lithological character when followed horizontally. A stratum of conglomerate becomes finer grained and gradually changes into sandstone, which shades off imperceptibly into slate, and slate into limestone, etc. Where the stratum is conglomerate, its thickness will usually be much greater and more variable than where it is composed of the finer sediments. The rapidity of these changes in certain cases is well shown by the parallel sections in Fig. 3. These represent precisely the same beds, as the connecting lines indicate, at points only twenty feet apart. [Illustration: Fig. 3.—Parallel sections showing rapid lateral changes in strata: _c._ clay; _s._ sand; _ss._ sandstone; _l._ lignite; _f._ fireclay.] When we glance at the conditions under which stratified rocks are now being formed, it is plain that all strata must terminate at the margin of the sea in which they were deposited, and in the marginal portions of that sea, especially, must exhibit frequent and rapid changes in composition, etc. The sediments forming the surface of the sea-bottom at the present time may be regarded as belonging to one continuous stratum; and it is instructive to examine a chart of any part of our coast, such as Massachusetts Bay, on which the nature of the bottom is indicated for each sounding, and observe the distribution of the different kinds of sediment. On an irregular coast like this, especially, the gravel, sand, and mud of different colors and textures, and the different kinds of shelly bottom, form a patchwork, the patches being, for the most part, of limited extent and shading off gradually into each other.