436 THE MEDALS OF CREATION. Cuap. XI. 
certain limestones, upon being broken or rubbed, are attri- 
butable to the evolution of sulphuretted hydrogen, from the 
animal matter which they contain. 
GEOLOGICAL DISTRIBUTION OF THE BIVALVE AND UNIVALVE 
Moriusca.—If the more rare and splendid organic remains 
may be regarded as the “ Medals of Creation,” the fossil 
testaceous mollusca, from their durability, numbers and 
variety, may be considered as the current coin of Geology. 
Occurring in the most ancient fossiliferous strata in small 
numbers, and of peculiar types,—becoming more abundant 
and varied in the secondary formations,—and increasing 
prodigiously, both numerically and specifically, in the ter- 
tiary, these relics are of inestimable value in the identifica- 
tion of a stratum in distant regions, and in the determination 
of the relative age of a series of deposits. To the solution 
of the former problem the sagacity of the late Dr. William 
Smith first suggested their applicability ;* while the idea, 
so happily conceived, and so philosophically carried out, by 
Sir C. Lyell, of arranging that heretofore chaotic mass of 
deposits, termed the Tertiary, into groups, by the relative 
number of recent and extinct species of shells, demonstrated 
the important aid to be derived from this class of organic 
remains, in the determination of some of the most difficult 
questions in geological science. 
Many useful tables have been constructed by Professor 
Phillips,t Sir C. Lyell, M. Deshayes, M. D’Orbigny, Prof. 
E. Forbes, and other eminent observers, to illustrate the 
geological distribution, in the several formations, of the 
genera and species of fossil shells hitherto described. To 
* See an interesting memoir of Dr. Smith, from the pen of his 
distinguished nephew, Professor Phillips. 
+ A Treatise on Geology; and Art. Geology, Encyclopeed. Metro- 
politana. 
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