118 DYNAMICAL GEOLOGY. 



inorganic nature ; and the chief source of climatal conditions through all 

 time since life began; which, further, in conjunction with the moon's 

 attraction, is the origin of the energy-distributing tidal wave, and also, 

 incidentally to the tidal movement, of tidal friction, with far-reaching, 

 adverse, and fatal results in the retarding of the earth's rotation. 



3. The earth's interior heat. 



Dynamical Geology is discussed beyond under the following heads : — 

 I. Chemical work, as a means of superficial changes. 

 II. Life, as a geological agent. 



III. The Atmosphere, as a mechanical agent. 



IV. Water, as a mechanical agent: under the subordinate heads of 



Water in general ; Fresh waters ; Oceanic waters ; Glaciers and 

 Icebergs. 

 V. Heat : under the heads of Sources of heat and their direct climatal 

 effects ; Expansion and contraction ; Igneous action ; Metamor- 

 phism; Veins and ore-deposits. 

 VI. Hypogeic work, or earth-shaping, mountain-making, and the 

 attendant phenomena. 



I. CHEMICAL WORK. 



Chemical work is given the first place, because superficial chemical 

 changes have been a prominent cause of the decomposition of rocks, and 

 thereby one of the producers of the earth, clay, and other fragmental ma- 

 terials which are worked into beds by the mechanically acting air and waters. 

 It is also a source of superficial rock formations of different kinds. Chemical 

 changes carried on at temperatures above the ordinary, as those of metamor- 

 phism, are not here considered. 



The following is the order of subjects : 1, Solution ; 2, Oxidation and 

 Deoxidation ; 3, Hydration, or the chemical absorj^tion of water ; 4, Carbonic 

 acid (CO2) and humus acids as geological agents ; 5, Action of siliceous solu- 

 tions ; 6, Chemical work of living organisms ; 7, Mechanical work of chemical 

 products ; 8, Concretionary consolidation. 



Of this large subject only a brief review of the more prominent facts is 

 possible in this place. 



Solution. 



The water descending in rains takes from the atmosphere its elements 

 (in the ratio of about two parts of nitrogen to one of oxygen) ; carbonic acid ; 

 some sulphates and ammonium nitrates, especially about cities where there 

 are coal fifes ; and three or four parts in 10,000 of sodium chloride or common 

 salt in the vicinity of the ocean ; besides atmospheric dust, enough of which 

 is from organic sources to make the waters offensive after standing a few 



