SciEXTiFic Lectures. 79 



on. During eveiy shower, or if vou will go back to the first cause 

 of all this, the evaporation of water by the action of the sun's rays 

 upon the ocean and upon the surface of the earth, lifting it into the 

 atmosphere and precipitating it again upon the surface, transfers the 

 loose materials into the smaller streams, thence into rivers, and thence 

 into the ocean, where they are spread out evenly from the facility of 

 their transportation by currents, the coarser materials being first 

 deposited, and then the finer. And the action of the frost annually 

 prepares these materials for the subsequent action of the rain. Tlie 

 water percolating into the crevices of the rocks freezes, and by its 

 expansion in freezing separates them, until year by year, more and 

 more of the rocky mass is broken down and the material prepared 

 to be transported by the rain-storms to the ocean. Regarding this 

 as an outline of the hills and valleys that you may find almost every- 

 where (di'awing upon the blackboard) ; penetrating beneath the sur- 

 face you find sand, gravel, and pebbles. But if you penetrate a Kttle 

 further, excepting in comparatively rare instances, you will find 

 beneath the accumulations of gravel, rocks lying in a nearly horizon- 

 tal position; beds of limestone, slate, and sandstone. These layers 

 of rocks have once been continuous, they have extended across the 

 valleys. Those portions of the strata, once filling the spaces between 

 the hills, have, by the action of water, been worn out. This essentially 

 horizontal position of the strata prevails in nine-tenths of the 

 area of the west. I will now give you an example of another position 

 of the strata. On the one side of the valley you may see the strata 

 inclining beneath the surface and on the other dipping in the opposite 

 direction. But still these strata have once continued across the 

 place of the present valleys ; and the same is essentially true in all 

 localities where the rocks are inclined, though they may dip very 

 steeply, as in your own neighborhood. These are nevertheless valleys 

 of erosion. The materials which once filled the valley, and made 

 these strata continuous, have been eroded and worn out by the action 

 of water and of ice. The fact is that the dip of the strata does not 

 change the law, for the beds have had this curved form (supplying the 

 connection), but the valley has been broken through wherever the 

 strata are wanting. It is a demonstrated law that whenever rocky 

 strata are bent upward they are weakened, so that the summit of the 

 arch is the weakest point. All the materials which are spread out 

 beneath the surface of the ocean, and all the rocky strata, as they are 

 originally deposited, are horizontal or very nearly so. This arching 



