PRliNCIPLES OF STRATIGRArmC CORRELATIONS 525 



ser limestone) fauna underlies the base of the typical Trenton section in 

 New York, and that it precedes the corresponding boundary also in Ken- 

 tucky. Moreover, a slight unconformity at the top of the bed containing 

 the Prosser limestone fauna in New York and Kentucky suggests a 

 hiatus that probably represents the middle and possibly also the upper 

 Galena. At least two-thirds of the Galena, plus the Kimmswick, there- 

 fore, belong between the base of the Prasopora simulatrix bed, which is 

 usually counted the basal member of the typical Trenton, and the top of 

 the Black Eiver group in Oneida County, New York. At present, then, 

 only the upper third of the Galena remains to be accounted for. Evi- 

 dence in hand suggests that a part (the Dubuque limestone) of this 

 upper division of the Galena is probably post-Trenton in age, but until 

 we know definitely what to do with it the post-Prosser part of the Galena 

 may be placed opposite the middle Trenton. It is to be noted, however, 

 that correlation results like these, while led up to by lithologic criteria, 

 are finally established mainly on faunal and diastrophic grounds. 



(5) Sequence of lithologic units. — Similarity in succession of t5rpes of 

 sedimentary rock, as of faunules, is of high value in correlating near-by 

 exposures and is of considerable use in comparing even widely separated 

 sections. Rarely this method may serve when the evidence of the fossils 

 is not by itself conclusive. So far as possible, however, the successive 

 steps of the correlation should be checked by the organic criteria; other- 

 wise great error may result. The Ordovician sections in central New 

 York and Kentucky afford a good illustration of the legitimate applica- 

 tion of the principle. Starting with the upper Stones River limestone, 

 which is practically the same in the two areas, the succeeding formation 

 is the Lowville limestone, which likewise agrees. Between the Lowville 

 and the Prasopora simulatrix zone (Wilmore limestone in Kentucky), a 

 shaly limestone easily recognized in both sections, come several thin, 

 late Black River and early Trenton beds that vary rapidly in thickness 

 and may be locally absent. Exact correlation of the succeeding Trenton 

 beds is more difficult, even with the help of abundant fossils, and the 

 lithologic criteria are quite as useful here as any of the other means. In 

 each of the two areas this part of the Trenton is marked by sea oscilla- 

 tion and consequent local peculiarities in sedimentation and faunal de- 

 velopment. The Trenton is followed by the widely overlapping black 

 and gray Utica shale. Only the upper part of this lithologically and 

 faunally characteristic zone reaches northern Kentucky, but it thickens 

 and is well known from deep-well records in Ohio. Lithologic and faunal 

 similarity continues in the two sections to the arenaceous top of the Lor- 



