390 H. S. WILLIAMS CORRELATION PROBLEMS 



ciples underlying our work, and look afield and take our bearings and see 

 whither we are drifting. 



It is quite evident that, the general practice of making correlations- and 

 locating our formations in horizontal parallels means to us that the cor- 

 related formations were deposited at synchronous intervals of time the 

 world over. The reconstruction of the world's geography is based largely 

 on the assumption that correlation of fossil faunas may be directly trans- 

 lated into contemporaneity, and we are apt to forget the many precon 7 

 ceptions involved in such translation. Let us call attention to some of 

 them: 



A. Identical species. — We speak of identical species. What does that 

 mean? I have listed among the identical species StropJiomena rhom- 

 boidalis Wahlenberg. This species is listed m formations ranging from 

 the Ordovician to the Carboniferous. 



Also I have listed Strophonella funiculata McCoy, the total range of 

 which in the Shropshire section is from the Tickwood beds underlying 

 the Wenlock limestone to the Lower Ludlow. It occurs in the ^Edmund 

 fauna, but I can find no evidence of its record from any other formation 

 in America. 



By the statistical method I have been obliged to use these two species 

 as units of equal value in determining the horizon. What is the differ- 

 ence between them that makes the one to be regarded as ranging through 

 four geological systems, the other to be restricted to a single fauna ap- 

 pearing in a limited area? Strophomena rhomboidalis presents in its 

 various forms as great an amount of morphologic diversity of characters 

 as do the genera Strophodonta, Strophonella, Leptostrophia taken alto- 

 gether, with all the many species described in their genera. 



In the comparison of the Edmunds with the transatlantic Wenlock, 

 both of these types of species are in evidence, but the fact is that the 

 larger number of the species found to be identical in the two faunas are 

 of long stratigraphic range; also of wide geographical distribution, and 

 those of the type of Strophonella funicula are rare. 



Not only is this true, but in making up a list of identical species there 

 is generally some degree of direct relationship between the species which 

 have a long stratigraphic range and identity in widely separate regions 

 of the earth. 



B. Plasticity and fixity of form. — The contrast between the two spe- 

 cies (mentioned as examples) does not end with the above statement. 



Such a species as Leptcena rhomboidalis has come to be regarded as a 

 single species not because all the specimens are morphologically alike, 





