THE ONYX MARBLES. 545 



Scborlemmer to be about 3 grams per liter of water, or 3 parts by 

 weight to 1,000. 



As has loug been kuowu, it is to the escape from solution of half 

 the combined carbonic acid, aided in some cases by the secreting power 

 of algous vegetation, that is due the deposition of the lime carbonate 

 in the form of sinters and tufas about the orifices of springs, in that 

 of scale in steam boilers and other vessels, or in the form of stalagmitic 

 and stalactitic deposits in caves. With its solvent power diminished 

 by the loss of the acid gas, the water deposits its load as rapidly as the 

 gas escapes. 



Kow, although we know this to be the i)rocess by whi(;h the calca- 

 reous deposition takes place, we do not know absolutely just what are 

 the conditions which control the character of the deposit as regards 

 compactness and condition of crystallization. Why in some cases the 

 deposit should be so compact as to be susceptible of an enamel-like 

 polish, and of such colors as to make a beautiful marble, or again light 

 and tufaceous like those now forming at the Mammoth Hot Springs in 

 the Yellowstone National Park, or the more compact lapis Tiburtinus 

 of Tivoli, Italy. Such synthetic work as has been done fails to throw 

 much light upon the subject. G. Rose* has shown that by humid 

 methods it is possible to produce out of the same solution crystals of 

 both aragonite and calcite, the one or the other forming according to 

 the temperature of the solution, Aragonite was formed exclusively by 

 a rapid evaporation of hot solutions, while calcite was produced from 

 similar solutions, both hot and cold. As the investigations here 

 chronicled have shown the onyx marbles to be almost invariably of 

 calcite, it is at once evident that we must look for other controlling 

 conditions than those of temperature for a satisfactory solution. Such 

 literature as bears upon the subject enables us, however, to draw, 

 from analogy, certain conclusions, and it may not be without interest 

 to refer here briefly to the expressed opinions of others. Thus Dr. 

 Edward Hitchcock, writing half a century ago, says:t 



"I have alluded to tbedepositiou ofm irble, or alabaster, by certain springs in the 

 vicinity of Lake Oroomiah (Persia). What is called the Tabrez marble has been 

 repeatedly described by travelers; but I doubt whether definite geological ideas have 

 yet been entertained respecting the mode of its formation. With the exception, 

 perhaps, of a deposit of travertine around Rome, in Italy, resembling statuary 

 marble, I am not aware of any case besides those around Oroomiah in vrhich the 

 most beautiful marble is produced by springs. The Tabrez marble is usually of 

 a yellowish or light blue color, perfectly compact, and so translucent that it is used 

 in thin slices for the windows of baths and other places, like the phengites of the 

 ancients. * * * The common opinion is that the springs now deposit it; but one 

 or two facts have led me to suspect that this may not be the case. Above the marble 

 there lies a deposit, several feet thick, of common tufa, or travertine. No\y, my sus- 

 picion is that this tufa is all the deposit which has been formed since the springs 



*Fouque and Levy, Synthese des Mineraux et des Roches. 



t Transactions of the Association of American Geologists and Naturalists, 1840-'42, 

 pp. 414-415. 



H. Mis. 184, pt. L' 3~i 



