422 THE CAMBRIAN. Ibuli..81. 



have the same order of superposition, as the conditions under which 

 they were accumulated are entirely dissimilar, and the proof of the 

 contemporaneity of deposition is not available. An exception to this 

 may occur where a stream passes through a lake and carries the fauna 

 and flora to the sea, where it is mingled with the marine fauna. 



II. ORGANIC REMAINS. 



There are several considerations always to be borne in mind in corre- 

 lating a formation on the evidence of its contained organic remains: 



(Yi) There can be no scientific or systematic paleontology without a 

 strati graphical basis. 1 



{b) The method of correlation by the comparison of fossils; or, as it 

 has been called, "matching," is the one that affords the best results. 

 It includes the comparison of species, genera, families, and the general 

 facies of a fauna. It is the basis of paleontologic correlation and geo- 

 logic classification of the sedimentary rocks, with the exception of the 

 stratigraphic and lithologic correlation of local formations. 



(c) In nearly every section of strata there are considerable thick- 

 nesses of beds, in which, no fossils are to be found. These beds occur 

 iu nearly every formation. The record of life, during these nonfossil- 

 iferous periods of sedimentation, is broken. 



(d) Many fossils have a great vertical range; and the occurrence of 

 a few species of this character without the presence of species of known 

 limited range can not be taken for the identification of any particular 

 formation. If the laws of constancy and variation of the wide-ranging 

 and long-enduring species are known, it may be possible to use them in 

 the correlation of minor formations, and thus create an exception to 

 the general rule. 



(e) All paleontologic reasoning is based upon known data. By the 

 discovery of a new grouping of fossils, or a different range of known 

 species, the identification of horizons may be materially modified. 



(/) In correlating a formation it is necessary to exercise caution be- 

 fore decidiug that the contained fossils belong to any specified zone of 

 the fauna characterizing a terrane or group of formations, and thus 

 locating the exact stratigraphic horizon of the formation. A fauna 

 may vary in its constitueut parts and afford different groupings of 

 species in localities not far distant. 



(g) All correlations between widely separated provinces are based 

 upon the general and not upon the specific character of the fauna; i. e., 

 it is the general assemblage of a fauna iu one basin that is compared 

 with that in another basin. 



Within limited areas the comparison of species and the grouping 

 of species suffice to* determine some particular horizon or formation. 

 In widely separated parts of the same area or province there will 



'Hall, James. On the Primordial fauna and Point Levi fossils. Am. Jour. Sci., 2d ser., vol. 31, 

 1861, p. 224. 



