RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 289 



the custom of geologists lias not been uniform as to the geueral desig- 

 nation given to eacli of the three grades represented by the three eol- 

 umnsofthe tables. In referring to and discussing these tables I shall 

 therefore apply the time term stage instead of age to systemsor divisions 

 of the second grade or middle column of the tallies and the term sub- 

 stage to the divisions of the third grade or left hand column. 



It will be seen by the foregoing statements that although the greal 

 scale in its entirety is comprehensive and trustworthy it is indefinite 

 and unequal in its divisions and subdivisions, respectively. The sig- 

 nificance of this indefiuiteness and inequality is made more or less 

 apparent in the discussions which are recorded on following pages. 



Although the purpose of this scale is the classification of the sedi- 

 mentary rocks of the earth its veal units are not physical but chrono- 

 logical. That is, formations are not the units in this case as they are 

 in the structural geology of districts or limited regions, but these units 

 are such divisions of time as are indicated by the successive changes 

 in the structure and character of the animals and plants which have 

 existed upon the earth from the beginning of life until now. It is true, 

 as has already been shown, that the chronological order of succession 

 of a few formations may sometimes be determined within limited regions 

 by means of their actually observed superposition, without reference 

 to the fossil remains which they may contain. Such a method of de- 

 termining that order, however, is wholly inadequate for general pur- 

 poses because opportunities for observing successive cases of super 

 position are comparatively rare and because formations never possess 

 any other than biological characteristics which originally could have 

 suggested the idea of their age with reference to the full course of 

 geological time. Therefore,a rational scheme of universal stratigraphic 

 classification can have no other than a biological basis. That is, its 

 real basis must be the evidence which fossils afford of the progressive 

 evolution of organic forms during the progress of geological time and 

 the various divisions and subdivisions of a resulting scale, while they 

 must of necessity be locally recognized with reference to formations 

 and systems, must coincide with widely recognizable secular stages in 

 the progress of thee volution. In short, the true basis of such a scheme 

 of classification is essentially a great biological idea to which material 

 expression is given by its application to the successive accumulations 

 of sedimentary deposits which now constitute the stratified rocks of 

 the earth. 



A comparison of the two preceding tables, although they represent 

 only an abridgment of the great scale, will show at a glance how well 

 the early geologists accomplished the work of constructing it. It will 

 be observed that after more than fifty years of active scientific investi- 

 gation the only essential changes that have been found necessary are 

 thefillingof a few gaps and the mure complete definition of the Cambrian 

 U. Mis. 114, pt. 2 19 



