Association of American Geologists and Naturalists. 15T 



and secondly, of a word or words descriptive of the ruling mineral 

 clmmcter of the rock; and to these is appended, when we wish. to 

 specify the type under which the formation is referred to, the name of 

 the district or place where it is so developed. Let me exemplify this 

 by one or two instances. The well characterized formation called in 

 the New York survey the Marcellus shales, is named by us the Post- 

 medidial older black slate, while the Genesee slate is called Postmedi- 

 dial newer black slate, and a member of the Clinton group of New 

 York, occuring there as a thin bed of brown and ponderous sandstone, 

 (seen on the Sequoit,) but expanded in Pennsylvania and Virginia into 

 an important mass having characteristic fossils and a maximum thickness 

 of two hundred feet, we propose to call the Levant iron sandstone. 



The nomenclature here employed for the designation of the strata, 

 is recommended we conceive by several features of obvious utility. 

 Being a nomenclature based on considerations of geological time, it 

 suggests at once in the names themselves the relative ages of the differ- 

 ent strata, thus defining the fundamental relationship of succession in 

 time, the only relationship between rocks which never varies. It has 

 thus the advantages of a numerical or ordinal designation, combined 

 with the descriptiveness which that has not. AVhile it conveys the fixed 

 relations of time, it expresses the accidental or local character of each 

 formation, and what is of much more importance, it signifies under 

 what particular type any special stratum is referred to, by introducing 

 into the name that of the district where the rock assumes the phases 

 treated of. Thus by the mere name assigned to each formation we 

 are reminded of all its most essential attributes, its age, its region, and 

 its mineral composition ; or in other words, what place among the other 

 strata it occupies — in what district we are describing it, and what its 

 composition is under its normal or typical development. One of the 

 most obvious defects in any nomenclature is a want to ready adaptibility 

 to new or abnormal relations and conditions of the objects named, and 

 in this unfortunate rigidity in the terminology of some of the sciences 

 we may discern a most influential barrier to their progress. In divising 

 the system proposed, we have aimed at uniting the power of representing 

 the fundamental or permanent relations of the objects named, and of 

 pliantly expressing their special deviations and gradations. In the 

 phenomena of mineral and Palseontological deviations of type lie con-" 

 cealed, we should remember, the very arcana of our science, secrets 

 which interpreted will give us the only insight we can hope ever to 

 procure into the actual state of our earth's surface in periods long re- 



