CLASSIFICATION OF IOWA EOCKS. 25 



a brief inquiry into his personal history wonld invariably 

 reveal the fact, that he had previously suffered pecuniary 

 disaster, and that he was indulging a morbid hope of retriev- 

 ing his fortunes by the discovery of mineral resources of 

 great value. 



3. CLASSIFICATION OF IOWA BOCKS. 



Every student of Natural Science will fully appreciate the 

 necessity of employing names for the objects he studies 

 that shall be entirely unequivocal in their meaning. But 

 owing to the ever varying characters and groupings of the 

 strata of the earth's crust, there seems to be a constant ten- 

 dency on the part of geologists, unless vigilantly guarded 

 against, to fall into a loose manner of naming them. For 

 example: the words epoch, period, age, group, formation, etc., 

 are, even by geologists, not unfrequently used in their general 

 sense rather than as names having an invariable meaning. 

 The condition of our language requires that for purposes of 

 scientific nomenclature we should employ names already in 

 common use, but the necessities of science require that we 

 should specialize those names for this purpose as far as pos- 

 sible, holding them strictly to one definite signification and 

 no other. I am well aware of the difficulty of establishing a 

 strict and uniform classification of the stratified rocks of any 

 region, but with the view of avoiding a loose or indefinite use 

 of terms, in designating certain of the assemblages of strata 

 of our Iowa and other rocks, I have decided to adopt in this 

 report the following classification^ This classification is fol- 

 lowed throughout the volume and an effort is made at all 

 times to designate the same object by the same name, and 

 to express the same idea by the same word or sentence. 



The term "formation" is restricted to such assemblages of 

 strata as have been formed within a geological epoch; the 

 term " group," to such natural groups of formations as were 

 each formed within a geological, period; and the term 

 " system," to such series of groups as were each formed 

 within a geological age. This arrangement has at least the 

 4= 



