198 THE AMERICAN MIDLAND NATURALIST 



paleogeographic difficulties, the paleontologic evidence is against 

 father than in favor of correlation of the Hackberry with the High 

 Point. 



If an equivalent for the Hackberry is to be found, it must be 

 looked for in the north and west rather than in the east. The simi- 

 larity of certain Hackberry species with forms from northwest 

 Canada and from western United States' is of interest. Thus 

 certain western forms of Spirifer whitneyi Hall are strikingly like 

 the Hackberry form which I have called S. whiineyi grada.MS. 

 But as yet nothing of sufficient importance to allow for correlation 

 has come up. Therefore, if we except time equivalency, we may 

 state without question that there is no described equivalent of the 

 Hackberry Stage. 



But is this formation a stage? Some writers think not; in one 

 publication of the Iowa Survey the formation was referred to as a 

 substage. ^ On the other hand, Calvin, Webster, Thomas, and others 

 have not hesitated to give the Hackberry the rank of stage or group. 



It is true that if we compare the thickness of the rocks of the 

 Hackberry with that of such formations as the Cedar Valley — 

 tO' take another Iowa Devonian formation — or with the great 

 thicknesses of some of the eastern formations, it makes but a poor 

 showing. But thickness of sediments does not make a stage, just 

 as it does not make a system. Fossils provide the evidence which 

 can be relied upon. 



The great difference of the Hackberry fauna from that of any 

 other described formation has been dwelt upon; that feature is of 

 importance in this connection as well as in correlation. The 

 radical change of the Hadkberry fauna from those preceding it in 

 the Iowa region is also of note. So far as known now, there is not a 

 single fossil species in the Sheffield formation which is found in the 

 Cedar Valley or Nora formations. So far as known, there is not a 

 single species common to the Hackberry and Sheffield formations. 

 The number of species common to the Hackberry -and the Cedar 

 Valley is small, and by careful study wall become smaller. 



On the other hand, if one is to consider the wealth of species, 

 3,nd the wealth of new species, in the Hackberry, the arguments 

 for giving the formation the rank of stage are greatly strengthened. 

 The number of formations in the PaleozDJ.c which can show as 



'See particularly Kindle; Bulletin 391, U. S. G. S, 

 'Eastman, la. Geol. Surv. Vol. XVII-I, pp. 45-50. 



