DISCRIMINATION OF TIME-VALUES IN GEOLOGY 575 



conglomerate may be relied upon for estimates of approximate 

 altitude. Nevertheless, in itself the formation possesses no 

 altitude value, it is only a conspicuous stratum in the region 

 where it appears, that now lies at a definite elevation above the 

 sea — the amount of the elevation must be determined by other 

 means. It may indicate an altitude already determined, but it 

 has no intrinsic altitude value, and is not a measure of altitude. 



In similar manner we may say that the geologic formation, 

 as a lithologic unit, was formed at some definite epoch of geo- 

 logic time which may be approximately measured by other 

 means ; but the amount of time from any datum point to the 

 time when it was formed is not in any degree indicated by the 

 lithologic or structural characters of the formations ; and in this 

 sense a formation may be strictly said to possess no intrinsic 

 time value, but to require the " paleontologic evidence" to prove 

 to what "period" of time it belongs. This conclusion might 

 almost be drawn from the statements regaiding periods in the 

 TentJi Animal Report (p. 65). 



It is, however, not necessary to remind geologists that fossils 

 possess a value as means of determining the time relations of 

 formations. But this time value of fossils will be better appre- 

 ciated if attention be given to the nature of the evidence furn- 

 nished by fossils regarding the period of time in which they 

 lived. 



TIME VALUE OF FOSSILS. 



Fossils derive their time -value froin the fact that the morphologic 

 characters presented by them are teinporary in nature. The form of 

 a trilobite, expressed in its various morphologic characters, was 

 constructed by organisms at a definite period of time in the 

 history of the earth ; so that the presence of a fossil trilobite 

 imbedded in a rock formation is direct evidence of the geologic 

 age in which alone the trilobite lived. Hence it is that for the 

 whole surface of the earth fossils become marks of the time 

 divisions and the means of correlating formations on a time- 

 basis ; this is the second method of correlation referred to above. 

 Identity of fossils is direct evidence of sameness of time. But 



