6 INTRODUCTION. 



composition. We may examine any given area, and find in it 

 nothing but one kind of rock sandstone, perhaps, or lime- 

 stone. In all cases, however, if we extend our examination 

 sufficiently far, we shall ultimately come upon different rocks ; 

 and, as a general rule, the thickness of any particular set of 

 beds is comparatively small, so that different kinds of rock 

 alternate with one another in comparatively small spaces. 



As regards the origin of the Sedimentary Rocks, they are 

 for the most part " derivative" rocks, being derived from the 

 wear and tear of pre-existent rock. Sometimes, however, they 

 owe their origin to chemical or vital action, when they would 

 more properly be spoken of simply as Aqueous Rocks. As to their 

 mode of deposition, we are enabled to infer that the materials 

 which compose them have formerly been spread out by the 

 action of water, from what we see going on every day at the 

 mouths of our great'rivers, and on a smaller scale wherever there 

 is running water. Every stream, where it runs into a lake or 

 into the sea, carries with it a burden of mud, sand, and rounded 

 pebbles, derived from the waste of the rocks which form its bed 

 and banks. When these materials cease to be impelled by the 

 force of the moving water they sink to the bottom, the heaviest 

 pebbles, of course, sinking first, the smaller pebbles and sand 

 next, and the finest mud last. Ultimately, therefore, as might 

 have been inferred upon theoretical grounds, and as is proved 

 by practical experience, every lake becomes a receptacle for a 

 series of stratified rocks produced by the streams flowing into 

 it. These deposits may vary in different parts of the lake, 

 according as one stream brought down one kind of material 

 and another stream contributed another material ; but in all 

 cases the materials will bear ample evidence that they were 

 produced, sorted, and deposited by running water. The finer 

 beds of clay or sand will all be arranged in thicker or thinner 

 layers or laminae ; and if there are any beds of pebbles these will 

 all be rounded or smooth, just like the water-worn pebbles of 

 any brook-course. In all probability, also, we should find in 

 some of the beds the remains of fresh-water shells or plants or 

 other organisms which inhabited the lake at the time these 

 beds were being deposited. 



In the same way large rivers such as the Ganges or 

 Mississippi deposit all the materials which they bring down at 

 their mouths, forming in this way their " deltas." Whenever 

 such a delta is cut through, either by man or by some channel 

 of the river altering its course, we find that it is composed of a 

 succession of horizontal layers or strata of sand or mud, varying 

 in mineral composition, in structure, or in grain, according to 



