304 HISTORICAL GEOLOGY 



the first locality. The coincidence of the lower fossils with 

 those of D-E of the earlier sections establishes our starting 

 point, and if the beds are conformable, so that we know 

 there was no interruption in deposition, we may now designate 

 the upper faunas as F, G, H, and /. This process of matching 

 and piecing out local sections has been carried on until we 

 have a nearly complete series of sections extending from the 

 present sediments back to rocks of very ancient times. 



The time value of unconformities. Thus far we have 

 considered only conformable strata, those which were laid 

 down one upon the other without interruption of any kind. 

 It often happens that the section under study contains an 

 unconformity. Without fossils this unconformity would tell 

 us that the region had emerged from the water after the 

 deposition of the lower beds, had been eroded, and again 

 submerged before the upper layers were formed ; but we 

 should not know how much time had elapsed while it re- 

 mained as land. Suppose, however, that the beds contain 

 fossils, and that the fauna just below the unconformity is like 

 B in our first section, while the one next above is identical 

 with G. We then know that faunas C-F are lacking and 

 that the region may have been land through all of the time 

 which was required for the gradual evolution of fauna B 

 into the new fauna G, or that it was land during let us 

 say the E and F periods and that the deposits of the C and D 

 periods were worn entirely away at that place. 



The geologic column. The completed series of sections, 

 often called the geologic column, is useful as a standard to 

 which we may refer any isolated rock formation in which we 

 can find the necessary fossils. Furthermore, an examination 

 of it showed very early that it contained certain natural divi- 

 sions which could be recognized all over the world, and from 

 time to time geologists have given to these divisions names 

 which are in general use. 



The dividing lines between the parts were usually the planes 

 at which sudden and marked changes in the fossils were 



