THE MUD FLAT. 45 



been brought to cur notice. In our Second and Third Talks 

 we have been led to speculate on the possible agency which 

 transported the bowlders from their northern home. We 

 have been thinking of glaciers as a satisfactory explanation ; 

 and this view was adopted by the late Louis Agassiz, and most 

 ably defended by him. Now, suppose there really was a vast 

 glacier covering the country as widely as the Drift at present 

 covers it. The ice must have melted; it is not here now. 

 Suppose it melted rapidly ; what enormous floods must have 

 been occasioned ! With what fury those floods rushed over the 

 country to the lower levels ! How they moved and mixed and 

 half assorted the sands and pebbles ! May not such a flood 

 have produced the results which we see in the semi-stratified 

 Drift? And then may not an excess of water have remained 

 in all the streams long after the southern portion of the 

 glacier had disappeared, and the semi-stratified Drift had been 

 put in place ? Would not such a state of weather as accom- 

 plished the melting of the ice have been somewhat like our 

 March and April weather, characterized by abundant rains? 

 Do we not find here good grounds for the building of a 

 theory of transported bowlders, half assorted sands and 

 flooded rivers? 



VIII. THE NluD 



SEDIMENTATION. 



WHEN the road-side pool left by the last shower dries 

 away a film of fine sediment remains. This once hung in 

 suspension in the water; it was gathered up from the land 

 by the eddies born of rain. We shall see that this simple 

 observation is the key to an explanation of many of the grand- 

 est facts in the world's history. 



A few years ago, in ascending the valley of the Aar, in 

 Switzerland, I enjoyed an extraordinary opportunity to ob- 

 serve the action of moving water. The Aar is a turbulent 

 stream issuing from the foot of the Aar glacier of the Jura 



