332 SECONDARY FORMATIONS. [Ch. XXIII. 



groups of strata above enumerated by us, names were given to 

 each, derived from the mineral composition of the rocks in 

 those parts of Germany, England or France, where they hap- 

 pened to be first studied. When it was afterwards acknow- 

 ledged that the zoological and phytological characters of the 

 same formations were far more persistent than their mineral 

 peculiarities, the old names were still retained, instead of being 

 exchanged for others founded on more constant and essential 

 characters. The student was given to understand, that the 

 terms chalk, green- sand, oolite, red marl, coal, and others, 

 were to be taken in a liberal and extended sense ; that chalk 

 was not always a cretaceous rock, but, in some places, as on the 

 northern flanks of the Pyrenees, and in Catalonia, a saliferous 

 red marl. Green -sand, it was said, was rarely green, and fre- 

 quently not arenaceous, but represented in parts of the south 

 of Europe by a hard dolomitic limestone. In like manner, it 

 was declared that the oolitic texture was rather an exception 

 to the general rule in rocks of the oolitic period, and that no 

 particle of carbonaceous matter could often be detected in the 

 true coal formation of many districts where it attains great 

 thickness. It must be obvious to every one, that inconvenience 

 and erroneous prepossessions could hardly fail to arise from 

 such a nomenclature, and accordingly a fallacious mode of rea- 

 soning has been widely propagated, chiefly by the influence of 

 a language so singularly inappropriate. 



After the admission that the identity or discordance of 

 mineral character was by no means a sure test of agreement or 

 disagreement in the age of rocks, it was still thought, by many 

 geologists, that if they found a rock at the antipodes agreeing 

 precisely in mineral composition with another well known in 

 Europe, they could fairly presume that both are of the same 

 age, until the contrary could be shown. 



Now it is usually difficult or impossible to combat such an 

 assumption, on geological grounds, so long as we are imper- 

 fectly acquainted with the geology of a distant country, inas- 

 much as there are often no organic remains in the foreign 



