Evolution of the Earth. 85 



intricate and diversified appearance. Yet beneath, this in- 

 finite diversity there is discoverable a certain order and re- 

 lationship, so that we may ascertain how these rocks came 

 into being, to what geological period they belong, their rel- 

 ative age, and we may sometimes determine the causes 

 which gave them their present local habitation. The great- 

 er portion of the earth's surface is covered by what are 

 termed stratified or sedimentary rocks ; that is to say, they 

 are rocks which are formed by the action of water, and are 

 found deposited in superimposed strata or layers. Some- 

 times these strata, in turn, are built up of an immense num- 

 ber of thinner laminae, or scales. This formation may 

 be observed at many places — as at Portland, Connect- 

 icut, in the sandstone quarries, and along the banks of Green 

 River in Utah Territory. Often an exposure of twenty feet 

 will reveal as many as from sixty to eighty superimposed 

 layers. Originally the stratified rocks must have been de- 

 posited horizontally. On the edges of cliffs and canons and 

 along the sides of quarries and mines, they may sometimes 

 be found and studied in this position. Often, however, they 

 have been tilted and contorted by the horizontal crushing 

 of portions of the earth's surface, so that their edges are ex- 

 posed in an inclined, sometimes in a vertical position ; occa- 

 sionally they are even tilted over so that their original posi- 

 tion is reversed. 



The question for us now to consider is, How were these 

 stratified rocks originally formed ? To answer this question, 

 Ave must observe more closely the action of brooks, rivers, 

 lakes, and of the ocean. When a current of water is mov- 

 ing swiftly along, its force is sufficient to carry with it not 

 only the finer particles of sand and soil, but also pebbles of 

 considerable size. The larger fragments are first deposited 

 in the shallow waters nearest the fountain-head of the 

 streams. The finer portions of sand are swept along by the 

 current, sinking more slowly and forming layers at the bot- 

 tom, while the still smaller particles of sediment travel 

 much farther and are deposited very gradually near the 

 mouths of the rivers or at the bottoms of larger bodies of 

 water. Doubtless many of you have followed the course 

 of a mountain stream, as I have done in the Catskills and 

 White Mountains, and observed the effects of this process. 

 In its earlier course, where the water shoots swiftly along 

 over rocks and ledges, what do you find in the bottom of 



