OUTLINES OF GEOLOGY. 



31 



the sea ; and the tower of Tignaux, erected on the shore so 

 late as 1737, is now a mile distant from it. 



The Delta of the Po has made still greater encroachments 

 on the Adriatic. The town of Adria, whence the sea de- 

 rived its name, a seaport in the time of Augustus, is now 

 twenty miles inland, and Spina, its predecessor, as a harbour, 

 was eleven miles from the sea when Adria was built. 



The Mississippi has protruded the cape in which it finds 

 its place of discharge, fifty miles into the Gulf of Mexico, 

 since its mouth was first discovered, and has advanced it 

 thirty miles since the city of New Orleans was built. Where 

 rivers, carrying large quantities of earthy matter, meet a 

 sea influenced by powerful tides, a continual struggle takes 

 place between the tendency of the one to form land and the 

 other to sweep it away. Islands are continually forming and 

 carried away, and, in many such cases, although changes are 

 going on, the extent of delta appears to have reached its limit. 



68. Beds of sand and gravel, and collections of solid masses 

 called boulders, are found in the same position in relation to 

 rapid streams, as beds of mud to those of more steady and 

 gentle current. With them we may class the downs or sand 

 hills, which are thrown up by the surf of the ocean, and the 

 beaches of sand and gravel which it forms as a boundary to 

 its own violence. 



69. In alluvial formations no other metal is found in any 

 quantity; except iron. This occurs in the shape of sand, and 

 of bog ore, in the lower parts of basins, occupied by turf or 

 alluvial mud. Some of its sands and gravels appear to con- 

 tain gold. The bog ore, however, is rather of chemical than 

 alluvial origin. 



70. Of organic remains, alluvium contains the relics of 

 man, and of all the animals which now inhabit the earth. The 

 latter are sometimes found in countries where they have long 

 ceased to exist ; as, for instance, the bones of the beaver in 



