IV PREFACE. 



dred feet above low water mark in the Ohio river at Cincin- 

 nati. In fact, of the two hundred and seventy-five species 

 common to other formations, nearly three fourths of that num- 

 ber would have a higher position in the Lower Silurian series of 

 strata than is indicated by their range in New York and Canada. 

 The difference of position of some of these species is remarkable, 

 and to illustrate the assertion, a few may be mentioned. Columna- 

 via halli, Nicholson (Columnaria alveolata, Hall), in New York is 

 a Black river Limestone species, while in Ohio, Indiana, and 

 Kentucky its range is above an horizon, equivalent to seven 

 hundred feet above low water mark at Cincinnati. In New 

 York, Protarea vetusta, Streptorhynchus subtentus, Orthis gubquad- 

 rata, Rhynchonella capax and dentata, are truly Trenton species, 

 while in the Cincinnati exposures not one has been found be- 

 low the six hundred feet level. 



Facts like these, showing the great intermingling of the spe- 

 cies of the Hudson River Group, Trenton, Black River, and 

 Birdseye Limestones, have made me doubt the propriety of 

 identifying any subdivision of the Cincinnati Group with any 

 of the Lower Silurian divisions of the East. As a provisional 

 compromise, I have appended a column of ranges. The plan 

 employed may be explained as follows : As is well known, 

 the thickness of that portion of the Cincinnati Group, above 

 low water mark in the Ohio River at Cincinnati, is about 

 eight hundred feet. This thickness, for convenience sake, I 

 have divided into eight equal parts, each comprising one hun- 

 dred feet of strata. All species restricted to beds below the 

 lowest of this series have been marked ''below 0." For exam- 

 ple, we may take Leptobolus lepis, Hall, range, — 1, signify- 

 ing, from lowest bed to one hundred feet above; Col- 

 umnaria alveolata, Goldfuss, range, 7 — 8, i. e., from seven 

 hundred feet above the low water mark layer to eight hun- 

 dred feet above the same stratum ; Orthis clytie, Hall, range, 

 below ; this species is found at Frankfort, Ky., in lower 

 strata than any exposed at Cincinnati. The range of each 

 species, as given, must be looked upon as approximate, since 

 I have no doubt that the majority of the limits will, in course 

 of time, be materially changed. The column was mainly pre- 

 pared for the convenience of collectors, and may be quite 

 useful to them. All that is claimed, so far as the accuracy 

 of the ranges is concerned, is, that the species either was 

 found by me within the limits stated, or I have used the in- 

 formation received from good authority that certain species can 

 be found at the given elevations in the series. 



