CHEMICAL HISTORY OF A VALLEY. 95 



Time went on; the waters of the sea gra- 

 dually went back, leaving more and more of the 

 bottom of the bay exposed, and the river had, 

 of course, now to flow through a longer channel 

 in order to reach the sea. As the waters re- 

 tired, they left a large part of the upper end of 

 the bay, where the river had emptied itself at 

 first, uncovered ; and here the river had left its 

 alluvium or sediment, spread out in a triangular 

 form, or like the Greek letter A, the point 

 representing the mouth of the river, and the broad 

 part or base, the spreading out of the sediment, 

 at first under the waters. 



At this time nearly all the soil contained with- 

 in this triangular piece, or, as it is called by 

 geologists, Delta, was " mineral soil ;" and was 

 obtained by the river, from the " waste " of the 

 hills and rocks. The sea appeared to retire, 

 leaving the bottom of the valley, for such it now 

 became, covered with the mineral soil, brought 

 down by the river. At first, this soil presented 

 the appearance of dried mud; and we should 

 have looked in vain for the soft brown layer, 

 which now covers the whole valley, from one 

 end to the other. This brown layer consists of 

 vegetable soil, and the vegetable soil had not then 

 been formed, for the waters had only recently 

 departed, leaving the ground dry. 



The seeds of plants carried by the wind, or 



