ALLUVIUM. RECENT FORMATIONS. 



777 



resembling the mud of our common chalybeate springs, observable in ponds, ditches, and 

 stagnant pools. Professor Bailey, of West Point in the United States, speaks of the elegant 

 fragile organisms of the bog iron ore, as occurring in immense quantities in the pools of 

 that neighbourhood, " the bottoms of which," he states, " are literally covered in the first 

 warm days of spring with a ferruginous- coloured mucous matter about a quarter of an 

 inch thick, which, on examination by the microscope, proves to be filled with millions 

 and millions of these exquisitely beautiful siliceous bodies. Every submerged stone, twig, 

 and spear of grass is enveloped by them ; and the waving plume-like appearance of a 

 filamentous body, covered in this manner, is often extremely elegant. Alcohol com 

 pletely dissolves the endochrome (soft colouring matter) of this species ; and the frus- 

 tules (siliceous shields) are left as colourless as glass, and resist the action of fire." Many 

 of the shields are only ^ o f the thickness of a human hair, one cubic inch of the iron 

 ore containing 1,000,000,000,000 of the skeletons of these living beings ! 

 4. Fluviatile estuary deposits ; deltas ; marginal sediments ; terraces in valleys. 

 Where rivers enter the sea with great force, the pebbly, sandy, argillaceous, and calca 

 reous sediments in their waters, with drift wood and organic exuviae, may be carried far 

 out into the heart of the ocean, and laid down upon its bed ; but generally speaking, 

 owing to a gentler flow, or to tidal action periodically arresting their current, deposition 

 proceeds rapidly at their mouths, giving rise to bars and shoals in estuaries, and to new 

 land in the form of gradually increasing deltas. The quantity of water discharged by 

 the Merrimac river, running by Lowell in Massachusetts, in 1838, was estimated by 

 Dr. Dana at 219,598,840,800 cubic feet. The quantity of matter chemically and mecha 

 nically suspended in the water, he rated, after many experiments, at 1,678,343,810 pounds 

 avoirdupois. In the Merrimac Print Works at Lowell, the annual amount of anthracite 

 coal used is about 5000 tons; and Dana calculated, that if the above mass of sediment 

 brought down by the river had been coal, it would have sufficed to supply the works with 

 fuel for the period of 167 years! The nature of the sediment transported by rivers 

 depends of course upon the countries through which they flow. After leaving the lake 

 of Geneva, the waters of the Rhone become chiefly charged with calcareous matter, the 

 deposits from which along the shore of the Mediterranean, at the mouth of the river, are 

 formed into solid beds of limestone. The mud of the Nile has been ascertained to consist 

 of ^ of argillaceous earth, about ^ of carbonate of lime, and ^ of carbon, besides silica, 

 oxide of iron, and carbonate of magnesia. Mr. Phillips remarks that materials of this 

 description may be deposited together ; but that, in the process of solidification, the 

 arrangement of the particles may be so influenced by peculiar attractions, as to become 

 rocks of the same character as the limestones, sandstones, and siliceous concretions of the 

 ancient formations. But rivers all along their course originate deposits which largely 

 modify their own channel ; and those that are periodically in flood contribute to a general 

 rise of the adjacent land, by annually spreading over it fresh layers of soil. 



The appearance of terraces or 

 shelves of no great breadth, but level 

 and distinct, covered with sand and 

 pebbles, is a very common feature of 

 river valleys. On this account they 

 are here noticed, though it is quite 

 as probable that they are the monu 

 ments of lacustrine, and in some in 

 stances of marine, as of fluviatile 

 action. Sometimes two or three occur 

 in succession above each other, as in the diagram, and unquestionably they indicate levels 



Section of a terraced Valley. 



a Upper terrace. 



b Lower terrace. 



c Existing river. 



