498 C. SCHUCHERT CHRONOLOGY ON BASIS OF PALEOGEOGRAPHY 



the diastrophic method. It will be seen from this arrangement that dias- 

 trophism as an aid in stratigraphic correlation is the last method to be 

 used ; but the time will come when it will be of primary importance, be- 

 cause it is the periodic rhythm of changes deep within the earth's mass 

 that brings about the elevation of mountains and the lowering and raising 

 of the oceanic strand-lines. Let us now consider these four methods in 

 the order given, since the sequence leads to ever more complicated methods 

 of research. 



TEE SEDIMENTARY METHOD 



Stratigraphy began in the discerning of the fact that superposed beds 

 and formations in a continuous sequence must have the older beds below 

 and the younger ones at the top. Therefore superposition of formations 

 imposes on the fossils a definite stratigraphic value and further indicates 

 the trend evolution has taken among the organisms. The nature of the 

 sediments taken by themselves can not, however, be used safely in corre- 

 lating widely separated localities having like formations, but may be of 

 considerable value in local work when the exposures are not far apart. 

 At all times the waters are depositing muds, sands, and limestones, and 

 as their sequence is variable in closely adjacent places, it follows that the 

 physical characteristics of the strata have the least value in correlation. 

 Limestones in heavy beds, however, have as single units the greatest value, 

 while sandstones and mudstones are the least usable in correlation. Even 

 though limestones are apt to be of wide distribution, these organic de- 

 posits make up only 5 per cent of the water-laid strata, and it therefore 

 follows that in actual practice they are of rare occurrence. On the other 

 hand, 80 per cent of the sedimentaries consist of mudstones, and as these 

 have the fewest fossils they are only of occasional value in correlation. 

 An identical succession of differing clastic deposits is, however, far more 

 reliable in correlation than the single units. 



Passage beds, or transitions from one kind of sediment to another, are 

 usually indicative of continuous deposition, and even a sharp contact 

 between differing strata may not be indicative of a break or disconformity. 

 On the other hand, a gradual transition does not preclude the possibility 

 of a break being present. Breaks occur less often in passing from a sand- 

 stone to a shale, from a shale to a limestone, or from a limestone into a 

 shale, and most often between a limestone and a succeeding sandstone. 

 Marine conglomerates are generally the initial strata of a transgressing 

 marine deposit, and are therefore of great value in directing attention to 

 the presence of a break. 



Sediments are of much service, however, in determining the ancient 

 shorelines. For this purpose the marine conglomerates of rolled and 



