ILUAM8.] CORRELATION OF FORMATIONS EAST AND WEST. 171 



In New York State, which had given the most perfect section of upper 

 Paleozoic formations, there appeared a complete series of deposits 

 distinguished by easily recognized differences in their lithologic char- 

 acters and in the fossils. The Coal Measures in Pennsylvania formed 

 an easily recognized datum above, and below the Devonian the sections 

 led by regular stages downward. 



As the eastern geologists went westward they attempted to correlate 

 the deposits discovered with the familiar standards of the Appalachian 

 province, the New York and Pennsylvania systems. The geologists 

 who begau their investigations in the Mississippi Valley and westward 

 correlated the formations with European standards, finding little to 

 help them in the eastern sections, and in the finer subdivisions classi- 

 fied them independently, as the New York geologists had already done 

 with their strata. 



On comparing notes, the geologists found that there were unmistak- 

 able differences in the rocks which occupied the same general intervals, 

 which were more extreme the more distant the contrasted sections 

 were from each other, and they assumed (a conclusion which was nat- 

 ural at that stage of progress in the science) that like differences 

 might be allowed for the faunas. This error was fatal and delayed for 

 years the acceptance of the correct interpretation which those who 

 depended upon evidence of fossils alone made in the early part of the 

 discussion. 



With the recognized variation in the composition of the strata, a 

 black shale which was present in a great number of the sections across 

 the country, and certainly below the Coal Measures and above Silurian 

 rocks, was seized upon as a common horizon by means of which the 

 sections of separate States might be tied together. The problem re- 

 garding the black shale consisted in the fact that in the standard sec- 

 tions of New York there were two black shales, the Marcellus and 

 Genesee, with the rich Hamilton fauna between them. When correla- 

 tions were followed across the States it was seen that no black shale 

 appeared in the northern part of the Mississippi Valley, but a Hamil- 

 ton fauna was found, and in the more southern sections little or no 

 trace of Hamilton faunas, but a single black shale. 



In the solution of this problem a study of the fossils alone finally 

 brought out the truth. 



A third problem came up, particularly concerning the sections of Ohio, 

 Michigan, and western Pennsylvania. With slight differences in the 

 characters of the deposits, on passing westward from the typical upper 

 Devonian of New York, there appear slight changes in the character 

 of the faunas. The question was, Is this a geographical modification, 

 or is it a change coordinate with sequence of time 1 For the Chemung 

 faunas do not extend westward under the Waverly, nor do the Waverly 

 faunas extend eastward over the Chemung. This problem is being 

 gradually settled by a minute study of the fossils, and the discovery of 



