GENERAL PRINCIPLES — CONCLUDED 



27 



species like those now living. The percentage of living species 

 becomes greater and greater as the present is approached, and on 

 this basis Lyell sub-divided a late period (Tertiary) into three 

 epochs. 



In any correlation problem the geologist strives to use as many 

 of the above criteria as possible, the certainty of the correlation 

 being more firmly established when several geological and paleon- 

 tological criteria are used together. 



Significance of Unconformities 



Thus far our discussion has been based largely upon the assump- 

 tion of conformable strata, but many times the succession of strata 

 (so-called " section") under study shows one or more unconfor- 

 mities. Since an unconformity represents a time of erosion, or 

 possibly a time of non-deposition of 

 sediments, it is obvious that it sig- 

 nifies an absence of both the strata 

 and life records for a greater or 

 lesser length of time. The missing 

 records for a given region can, how- 

 ever, generally be found by going 

 to some other locality where deposi- 

 tion of sediments was not inter- 

 rupted at the time when the uncon- 

 formity was being produced. 



Without the aid of fossils, in the 

 ordinary case of unconformity, we 

 could tell that the land emerged 

 above water, was eroded, and again 

 submerged, but we could not tell how much time the erosion 

 interval involved (Fig. 18). But by noting the fossils in the 

 youngest strata just below the eroded surface, and in the oldest 

 strata just above it, we could tell what epochs or periods are 

 represented by the erosion interval by a comparison with the 

 standard geologic section of the world (see table near the close of 

 this chapter). 



Because the fossils immediately above and below the line of a 

 profound unconformity show such marked differences, the earlier 

 geologists were misled into thinking that each great unconformity 



Fig. 18 

 Diagram to illustrate the sig- 

 nificance of unconformities. 

 The lower strata (A) were 

 folded, raised above water, 

 eroded and then submerged, 

 after which the upper strata 

 (B) were deposited and then 

 tilted. (W. J. M.) 



