678 DYNAMICAL GEOLOGY. 



4. FORMATION OF SEDIMENTARY STRATA. 



In recapitulation, sedimentary strata have been formed — 



1. By the waters of the sea. 



(1.) Through the sweep of the ocean over the continents when 

 barely or partly submerged, — making (a) sandy or pebbly deposits near 

 or at the surface where the waves strike, or at very shallow depths 

 where swept by a strong current ; (b) argillaceous or shaly deposits 

 near or at the surface, where sheltered from the waves, and also 

 at considerable depths out of material washed off the land by the 

 waves or currents ; (c) but not making coarse sandy or pebbly de- 

 posits over the deep bed of the ocean, as even great rivers carry 

 only silt to the sea ; and not making argillaceous deposits over the 

 ocean's bed except along the borders of the land, unless by the aid 

 of a river like the Amazon, in which case, still, the detritus is 

 mostly thrown back on the coast by the waves and currents. 



(2.) Through the waves and currents of the ocean acting on the 

 lorders of the continent with the same results as above, except that 

 the beds have less extent. 



(3.) Through living species, mainly Mollusks, Radiates, and Rhi- 

 zopods, affording calcareous material, and Diatoms and some Proto- 

 zoans, siliceous material. All rocks made of corals and the shells 

 of Mollusks, excepting the smallest, require the help of the waves 

 at least to fill up the interstices (see p. 619) ; but Rhizopods and 

 siliceous Diatoms may make rocks in deep water, by accumulation, 

 which are in no sense sedimentary. 



2. By the waters of lakes. — Lacustrine deposits are essentially like 

 those of the ocean in mode of origin, unless the lakes are small, 

 when they are like those of rivers. 



3. By the running ivaters of the land. — (1.) Filling the valleys with 

 alluvium, and moving the earth from the hills over the plains. 

 (2.) Carrying detritus to the sea or to lakes, to make, in conjunc- 

 tion with the action of the sea or lake waters, delta and other 

 sea-shore accumulations. 



4. By frozen waters. — (1.) Spreading the rocks and earth of the 

 higher lands over the lower, and, in the process, bearing on blocks 

 of great size, such as cannot be moved by other means. (2.) Carry- 

 ing rocks and earth from the land to the ocean, either to the 

 sea-shore, making accumulations in lines or moraines, or to distant 

 parts of the ocean, as from the Arctic to the Newfoundland Banks; 

 and thus contributing to deep or shallow water or shore sediment- 

 ary accumulations, distinguished by the irregular intermingling ol 

 huge blocks of stone, pebbles, and earth. 



