3 68 PLEASANT WA YS IN SCIENCE. 



new igneous rocks are produced in the bowels of the earth, 

 but we cannot watch the progress of their formation ; and as 

 they are only present to our minds by the aid of reflection, 

 it requires an effort both of the reason and the imagination 

 to appreciate duly their importance." But that they are 

 actually of extreme importance, that in fact all the most 

 characteristic features of our earth at present are due to the 

 steady action of these two causes, no geologist now doubts. 

 I propose now to consider one form in which the earth's 

 aqueous energies effect the disintegration and destruction of 

 the land. The sea destroys the land slowly but surely, by 

 beating upon its shores and by washing away the fragments 

 shaken down from cliffs and rocks, or the more finely divided 

 matter abstracted from softer strata. In this work the sea is 

 sometimes assisted by the other form of aqueous energy the 

 action of rain. But in the main, the sea is the destructive 

 agent by which shore-lines are changed. The other way in 

 which water works the destruction of the land affects the 

 interior of land regions, or only affects the shore-line by 

 removing earthy matter from the interior of continents to the 

 mouths of great rivers, whence perhaps the action of the sea 

 may carry it away to form shoals and sandbanks. I refer to 

 the direct and indirect effects of the downfall of rain. All 

 these effects, without a single exception, tend to level the 

 surface of the earth. The mountain torrent whose colour 

 betrays the admixture of earthy fragments is carrying those 

 fragments from a higher to a lower level. The river owes its 

 colour in like manner to earth which it is carrying down to 

 the sea level. The flood deposits in valleys matter which 

 has been withdrawn from hill slopes. Rainfall, acts, how- 

 ever, in other ways, and sometimes still more effectively. 

 The soaked slopes of great hills give way, and great landslips 

 occur. In winter the water which has drenched the land 

 freezes, in freezing expands, and then the earth crumbles 

 and is ready to be carried away by fresh rains ; or when dry, 

 by the action even of the wind alone. Landslips, too, are 

 brought about frequently in the way, which are even more 



