STRATIFIED ROCKS. 6/ 



clearness, and definitencss of thought and language, to retain 

 the nomenclature now in use for the classification of rock 

 formations and to apply names derived from the fossils to the 

 time-divisions, since the fossils are the means by which the 

 time-divisions are recognized. 



A geological formation, made up of clastic fragments of 

 other rocks, has in itself nothing by which to determine its 

 time-relations; it is only its position, geographical and strati- 

 graphical, in relation to underlying or superimposed strata 

 that indicates its relative time-relation ; when considered 

 abstractly, or irrespective of its position, it loses its time-indi- 

 cating characters. 



Strata Parts of a Geological Formation, Fossils the Marks of a 

 Geological Period. — It is not scientific, therefore, to speak of a 

 rock or stratum as belonging to a particular period, the rock 

 belongs to a formation. The fossil imbedded in it, however, 

 does belong to the period, is characteristic of the period, and 

 thus, in nomenclature, it is actually taken as the mark of the 

 time-division. Just as we speak of the Chemung group, as 

 the name for the upper Devonian rocks of New York State, 

 so with like propriety we may say the disjmicta epoch, or the 

 epoch of the Spirifcra disjiincta and the fossils associated 

 with it ; and for the same reason. The application of Che- 

 mung to the group is appropriate, because one of the most 

 typical outcrops of the rocks so named is along the valley of 

 the Chemung River, at Chemung Narrows, in southern New 

 York. Not that it is not exhibited elsewhere, and not that 

 it is all exhibited at Chemung Narrows, but the group of 

 rocks of which the cliffs at Chemung are a good example 

 is appropriately and distinctly defined by the name. So to 

 call the epoch the disjuncta epoch is appropriate, because the 

 Spirifera disjuncta is a characteristic shell in the fauna of the 

 epoch, and the designation disjuncta as a specific name is 

 permanently applied to those characteristics of the genus 

 which are peculiar to the closing part of the Devonian age, in 

 all regions from which the fossil has been obtained ; and al- 

 though not the only fossil, and this one not always present, 

 still it may be used whenever found as indicative of the time- 

 epoch which is so named. 



