DEFINITIONS AND LAWS OF GEOLOGY. 15 



purpose ; it can never mislead as to the mineral structure or relative position of the 

 strata, and it indicates the typical locality of the exposure. Sandstones, conglom- 

 erates, limestones, and shales, occur in nearly every Group, and for this reason 

 geological subdivisions can not be established upon the mineral or chemical char- 

 acters of the rocks. The rocks which form these Groups are composed of a few 

 simple minerals, which are repeated over and over again in the different layers, but 

 not in chronological succession, nor in any other way indicative of age or position. 

 To speak of a rock as a limestone, sandstone, shale, slate, or clay, conveys no idea 

 of its geological age or place. It is merely the expression of a mineralogical 

 character. 



§ 24. Prof. Rogers conceived the idea of improving the nomenclature of the 

 palaeozoic rocks by dividing them into fifteen parts, and giving them names signifi- 

 cant of their relative ages. This he did by using words suggestiug metaphorically 

 different parts of a day, as follows : Primal, Auroral, Matinal, Levant, Surgent, 

 Scalent, Premeridian, Meridian, Post Meridian, Cadent, Vergent, Poneut, Vesper- 

 tine, Umbral, and Serai, meaning respectively the formations of the Dawn, Day- 

 break, Morning, Sunrise, Mounting Day, Climbing Day, Forenoon, Noon, After- 

 noon, Declining Day, Descending Day, Sunset, Evening, Dusk, and Nightfall. 

 Unfortunately for his attempt to substitute another for the geographical nomencla- 

 ture then quite well established and susceptible of indefinite expansion, without the 

 use of conflicting terms or words that could mislead the student, there were several 

 extensive Groups of rocks full of the remains of animal life, then unexplored, and 

 consequently quite unknown to his system. For obvious reasons the nomenclature 

 suggested by Mr. Rogers has not been adopted. 



§ 25. The words series, strata, layer, deposit, zone, bed, horizon, period, age, 

 epoch, and era are not technical names, but are used in geological descriptions, be- 

 cause expressive and convenient. Each Group must, in all cases, depend upon the 

 palseontological characters, and can never rest upon the structure of the rocks. 

 When properly defined, it is established, and no one has a right to substitute another 

 name for it, nor to propose a name, simply because of inability to properly distinguish 

 it at a particular locality. For example, the Trenton, Utica, and Hudson River 

 Groups had been long established, when some one, being unable to distinguish the 

 Utica in the vicinity of Cincinnati, and not knowing whether the rocks are Trenton 

 or Hudson River, proposed to call the exposure the " Cincinnati Group." The 

 black slate, which characterizes the Utica in New York, does not exist at Cincinnati, 

 though calcareous slates and shales of the same age do, and they contain Triarthrus 

 becki, Leptobolus lepis, and other characteristic fossils, while the Hudson River is 

 plainly distinguishable above, and the Trenton as readily determined below. If the 

 Utica had thinned out in its extension westward from New York before reaching 

 Cincinnati, there would have been no excuse for calling the Trenton or Hudson River, 

 or both of them together, by a new name, nor is there any excuse for so doing when 

 the Utica is easily distinguished. 



§ 26. Another kind of synonymy to be deplored exists where a Group has 

 been named and thoroughly defined, and for some trivial reason, the geologists of 

 another locality use another name for rocks of the same age without regard to 

 priority in nomenclature. For example, the Calciferous Group was established and 

 defined so as to include rocks other than Calciferous sandrock, and ten years after- 



