walcott.) CORRELATION. 401 



has never been known to occur in any other geological position, is represented by a 

 drab-colored shale iu the vicinity of Cincinnati, Ohio. There also the equivalent or 

 continuation of the black limestone of Trenton Falls is of a gray or pale hue. and 

 could be known as the same rock only by its organic reliquiae. The Caradoc sand- 

 stone series of Wales is represented in New York by limestone and slate in proportion 

 equal to the arenaceous strata. 



Hall — The desirability of naming formations from typical exposures 

 to serve as standards of comparison for the purpose of correlation was 

 enunciated by Prof. James Hall very clearly in 1839, as follows : 



Everyone who has studied rocks even partially is aware of the insufficiency of 

 mineral or lithologieal characters for giving nomenclature, and the many errors into 

 which he may be led, whether in his own researches or by the mistakes of others. So 

 likewise in the present state of our knowledge we are unable in all cases to give 

 'names from fossil characters; for, though without doubt every group embraces its 

 peculiar fossils, yet in all localities these may not be so marked as to excite attention, 

 and in some may possibly be absent. It thus becomes a desideratum to distinguish 

 rocks by names which can not be traduced, aud which, when the attendant circum- 

 stances are fully understood, will never prove fallacious. The basis of this nomen- 

 clature is derived from localities, and the rock or group will receive its name from 

 the place where it is best developed. 1 



He then proceeds to name the "Rochester shale" from Rochester, 

 New York, and the Lockport limestone from Lockport, New York. 



In 1843, after stating the general results of an examination of a sec- 

 tion southwest from Cleveland to the Mississippi River, he wrote: 2 



From the facts here stated the conclusion seems unavoidable, that the character of 

 fossils is, or may be, as variable as lithologieal characters; in fact, that the species 

 depend in some degree upon the nature of the material among which they lived. 

 Fossil characters therefore become of parallel importance to the lithologieal; and, 

 in order to arrive at just conclusions, both must be studied in connection, and locali- 

 ties of proximity examined. In the case of the Hudson River group of shales and 

 sandstones, in passing from New York to Ohio, the lithologieal character is almost 

 entirely changed ; and at the same time also the most prominent and abundant fossils 

 are unlike those of the group in New York. More careful examination, however, re- 

 veals the fossils which characterize this group at the East, and also at the same time 

 some obscurely similar lithologieal characters. Similar lithologieal changes, accom- 

 panied by like changes in fossils, occur in more limited districts within the State of 

 New York. 



The most marked and important changes, however, appear to be in the higher rocks 

 of the New York system. The Hamilton group and Marcellus shale, which in New 

 York have a thickness of 1,000 feet, have diminished to 100 where last examined; and 

 from being the group most prolific in fossils, as it is in New York, it has become en- 

 tirely barren of them. The rocks forming the Portage and Chemung groups, which 

 in their greatest development in New York are scarcely less than 3,000 feet in thick- 

 ness, and in Pennsylvania much more, have in Indiana diminished to as many hun- 

 dred. The upper of these groups, from being extremely fossiliferous, has become 

 almost destitute of these characters, so that, at the farthest extreme examined, they 

 furnish but an equivocal guide. In these groups lithologieal character is more per- 

 sistent than fossils, and it requires aknowledge of the superposition to identify them 

 satisfactorily. The greater thickness of these sedimentary deposits, and the greater 



1 Third Annual Report of the 4th Geological district of New York. Third Annual Report of the 

 Geological Survey of New York. Albany, 1839 ; pp. 288, 289. 



2 Notes explanatory ot a section from Cleveland, Ohio, to the Mississippi River, in a southwest di- 

 rection ; with remarks upon the identity of the western formations with those of New York. Assoc. 

 Am. Geol. Trans., 1843, pp. 289-291. 



Bull. 81 26 



