GEOLOGICAL NOMENCLATURE. 193 



more ancient formations, and each, imbedding 1 in its sediment 

 the characteristic shells, fishes, reptiles, and plants, which were 

 either washed into, or once lived in the ancient sea, of which it 

 formed the bed. 



The names which have been given to the different geological 

 formations must be received with some caution, for they are not 

 always indicative of formations identical with those from which 

 the name was derived. Many of these names are borrowed from 

 place^ thus, we read of the Jura limestone, the Kimmeredge 

 clay, Oxford clay, Purbeck marble, Portland rock, and Potsdam 

 sandstone. These names, referring to the stratum of a known 

 locality, were good so far as "an identity with that stratum can 

 be traced, but from the nature of the case, this is often incom- 

 pletely done, and hence the names necessarily cease to be defi- 

 nite. Many of the English provincial names are still retained, 

 though .very uncouth and harsh sounding, thus Geologists often 

 employ the terms Cornbrash, Lias, Gault, Coral Rag, and many 

 others which have no systematic signification. 



Descriptive names applied in Geology are also defective, and 

 when employed, no scrupulous regard must be had to their appro- 

 priateness. " The Green Sand may be white, brown, or red ; 

 the Mountain Limestone may occur only in valleys ; the Oolite 

 may have no roe-like structure ; and yet these may be excellent 

 geological names, if they be applied to formations, geologically 

 identical with those which the phrases originally designated." 

 The term Oolite is an instance where a descriptive word has be- 

 come permanent, and in like manner the term proposed by Mr. 

 Murchison, for the transition series of rocks, which, from being 

 distinctly marked in South Wales, he calls Silurian, from th$ 

 name of the ancient inhabitants, is in many respects excellent, 

 The terms employed by Mr. Lyell, before mentioned, as divisions 

 of the Tertiary formation, viz : Pliocene, Miocene, and Eocene, 

 according to the percentage of recent shells, being founded upon 

 a more natural'distinction will undoubtedly come into general use, 

 but even these are to be used with caution, and not allowed to set 

 aside the indications drawn from the natural relations of the strata. 



