30 



OUTLINES OF GEOLOGY. 



a new growth of turf. Trees are frequently imbedded in 

 turf bogs, and, of some of them, trees form the principal part. 



66. The mechanical formations of alluvial origin are 

 wholly produced by the action of water. They are always 

 found in the neighbourhood of existing water-courses, and in 

 positions either actually within their present sphere, or such 

 as may have been reached by them, without any change in 

 the general circumstances of the earth's surface. They are 

 made up of beds of sandy, clayey, or marly mud, of sand, 

 gravel, and pebbles, and of collections of rolled masses or 

 boulders. The mud is frequently coloured black or brown 

 by the carbon arising from the decay of organic matter ; and 

 it frequently exhales offensive gases, which are unwhole- 

 some. This formation covers the valleys of streams form- 

 ing what are called by our settlers, bottom lands — extends 

 into the sea at the mouths of rivers, forming banks and shoals, 

 and constitutes deltas, where rivers of the largest size meet 

 the ocean. 



67. Deltas have been formed, or have much increased, 

 within historic periods, and are still, in many cases, extend- 

 ing themselves into the ocean. The space now occupied by 

 the Delta of the Nile is represented by Herodotus to have 

 been a gulf in the time of Moeris, and we can easily believe 

 how rapidly it must have formed, when we observe the vast 

 quantity of sediment, about one part in four, contained in the 

 water of the Nile. At present, however, it is no longer on 

 the increase, and an obvious reason has been found why it 

 should not ; for, it has been protruded so far as to meet a 

 rapid current, which sweeps from the Straits of Gibraltar 

 along the shores of Africa. 



At the mouth of the Rhone, a Roman road, which once 

 followed the line of the beach, is now several miles inland. 

 Notre Dame des Portes, a harbour A. D. 808, is now a 

 league ; Psalmodi, an island A. D. 815, two leagues from 



