wALCOTT.l CORRELATION. 403 



subject and aid in deciding the true cause and amount of variation in paleozoic char- 

 acters when examined over wide districts. Wo shall doubtless be led eventually to 

 see in all these changes the influences of depths of water, distance from or proximity to 

 land, and the influence of the nature of that deposit which formed the bed of the ocean, 

 on which the animals lived. Every one of these causes, and perhaps other minor ones, 

 have influenced the preseut character and condition of our older fossiliferous deposits. 

 All these circumstances influence the organic productions of our present ocean, what- 

 ever may be the climate ; and we have every proof that the same causes operated in 

 this ancient sea, where, although depth and temperature may have been more uniform, 

 yet these could not have been paramount to all other influences. 1 



He then proceeds to correlate the strata of the Mississippi Basin with 

 those of the New York section by means of the contained fossils, and 

 where possible the order of stratigraphic succession. 



Emmons. — In a geological report of the midland counties of North 

 Carolina, Dr. E. Emmons, in 1856, speaks of the classification of the 

 sediments and says in this connection : 



Superposition is, however, the highest proof of age, the oldest occupying the in- 

 ferior position. 



The bearing which fossils have to any scheme of classification which has been 

 proposed can be understood only by a knowledge of the following laws: (1) That 

 species or kinds have had a limited duration ; (2) that there has been a succession of species; 

 and (3) that the species of one period, and which have become extinci, hare never lived in 

 any future period. The utility of the knowledge of fossils is based on these three, laws. 

 This knowledge is particularly useful in comparing rocks which are widely separated 

 from each other, or in those cases where direct superposition can not be observed. If, 

 for example, certain rocks in Canada furnish a group of fossils similar to those of a 

 given series in Tennessee, the inference would be that they belonged to tlie same 

 period and hence occupy the same geological position ; or, if we compare the fos- 

 sils of the coal formation of England and America it will be found that they are 

 almost identical ; and it is proved also that the position relatively is the same in 

 both countries, though separated from each other 3,000 miles. 2 



The extension of the New York section to the south along the line of 

 the Appalachian Mountains was principally the work of the Rogers 

 Brothers and Prof. Safford. 



Rogers. — As geologists Messrs. H. D. and W. B. Rogers traced the 

 the formations by stratigraphic continuity, lithologic characters, serinl 

 relation, and the presence of similar fossils as in the Trenton limestone, 

 etc. I have not met with a statement of their views of geologic correlation 

 except in incidental remarks. In a reply made to Prof. James Hall, 

 who had congratulated the Profs. Rogers that they had borne such able 

 evidence to the value of organic remains in determining the age of 

 rock, Prof. W. B. Rogers said : 3 



That they had not been understood on this point; they had followed out the in- 

 tricate structural geology of Pennsylvania and Virginia, relying chiefly on litholog- 

 ical characters, and had found to their great gratification that they had been working 

 in parallel planes to the New York geologists, whose labors among the regular and 



1 Nature of the strata and geographical distribution of the organic remains in the older formations of 

 the United States. Boston Jour. Nat. Hist., vol. 5, 1845, pp. 2, 3. 



'Geological report of the midland counties of North Carolina. New York and Raleigh, 1856, p. 24. 



3 A system of classification and nomenclature of the paleozoic rocks of the United States, with an 

 account of their distribution, more particularly in the Appalachian Mountain Chain. Am. Jour. Sci., 

 vol. 47, 1844, p. 112, 



