64 WALKS AND TALKS. 



of these have we ever seen making ; but I think we have seen 

 a process similar to rock-making in the beds of alluvial mat- 

 ter deposited by an overflowing stream. In traveling down 

 the lower Mississippi, we can see from the deck of the steamer 

 that the material of the alluvial banks is horizontally strati- 

 fied. More strictly we should say that it is laminated ; but 

 the nature of the geological work is the same in either case. 

 Now, if those alluvial banks should become firmly consoli- 

 dated, they would present the appearance of some of the rocky 

 cliffs those in Watkins' Glen, for instance. You have also 

 learned how large quantities of sediments borne down by 

 rivers are carried out to sea many miles, and slowly deposited 

 on the ocean's bottom. These deposits must necessarily be in 

 layers, each of which is spread evenly over the bottom. You 

 remember that the distance to which materials of a certain 

 degree of coarseness may be carried before sinking to the 

 bottom, depends on the velocity of the motion of the water. 

 At a certain place in the sea the velocity is undoubtedly more 

 rapid at one time than another. The motion is caused by 

 winds, by tides, and by currents. Therefore, a coarser sheet 

 of materials will be laid down at one time, and a finer sheet at 

 another. The alternations of coarser and finer render the 

 bedded arrangement conspicuous. Very likely the colors of 

 the sediments will vary also ; since, from one direction, they 

 may be supplied by pulverized limestone, from another by 

 pulverized sandstone, and from another by pulverized shale, 

 which may be blue, red, or black. We noticed, too, in our 

 walk under the sea, that sedimentary materials are spread 

 over all the slope of the ocean's floor, within fifty or a hun- 

 dred miles of the land often much farther, if the shore is 

 "shelving" or the currents are favorable. 



These various indications compel us to adopt the conclu- 

 sion that water has been the agent by which the materials of 

 the stratified rocks have been spread out in broad beds or 

 strata. But, though river overflows must leave the sediments 

 in a bedded condition, these beds are not exactly like those 

 seen in great formations of limestone and sandstone. Kiver 



