^TRATIGRAPHIC TAXONOMY 585 



in the character of sediments or by the structural relations of the beds. 

 At their best the hitherto prevailing methods of drawing strati graphic 

 boundaries often obscure general as well as local geologic history, because 

 the facts relating to changes in sedimentation are not brought out in 

 their proper relationshij). In many such cases, referring more particu- 

 larly to formations, the so-called "transition beds" have been referred to 

 either the top of the lower or the base of the upper formation without 

 considering the probability of a stratigraphic hiatus beneath them. If 

 the transition beds happen to be initial deposits of the upper formation, 

 but agree lithologically more closely with the underlying formation, to 

 which therefore they have been referred, the geologically important break 

 is ignored while a relatively insignificant boundary is recognized. Many 

 instances of this kind might be cited. In the case of larger divisions it 

 happens occasionally that the fauna of a certain formation consists al- 

 most entirely of species whose alliance, for instance, are strongly Silurian, 

 while elsewhere a synchronous or even a younger faima is dominated by 

 species recalling only Ordovician types. According to the old method 

 one would be referred to the Silurian, the other to the Ordovician. On 

 similar grounds contemporaneous formations have been referred to dif- 

 ferent series or groups. Oftener, however, general similarity in faunas 

 found in locally superposed formations, or in geographically separated 

 formations, which on the ground of such similarity may have been cor- 

 related, has caused their reference to the same group or series when in 

 fact they belong to distinct groups or series. Examples of all of these 

 conditions have been discussed and the difficulties explained in preceding 

 parts of this work. And many more are indicated on the correlation 

 tables appearing farther on. At the risk of repetition a few may be 

 cited here. 



The line between the Cambrian and the Ordovician, in areas contain- 

 ing a considerable development of these broadly conceived systems,. was 

 either not drawn at all or it was placed variously by authors at one or an- 

 other horizon in the great intervening thickness of beds that is sharply 

 defined above and below, and in fact includes representatives of two in- 

 termediate systems. In the case of the Siluro-Ordovician boundary it 

 was drawn in N'ew York at the base of the Oswego sandstone, a formation 

 that is undoubtedly older than the Eichmondian in the Ohio and Missis- 

 sippi valleys, where the latter deposits were included in the Ordovician. 

 However, in both of these cases the error lay not so much in method a9 

 in correlation. The same may be said of the Devono-Waverlyan bound- 

 ary, which was drawn higher in the scale in Tennessee and Kentucky 



XXXIX — Bill. Gkol. Soc. A>r., Vol. 22, 1010. 



