PRIXCIPLES OK STRATIGRArHIC CORRELATIONS 541 



Valley south of central Virginia; also in the Wills Creek, a Cayugan 

 formation in Maryland and Pennsylvania. 



(11) Correlation by evidence of progressive siihraergence. — Conglom- 

 erates and the various other initial deposits described on pages 454 to 456. 

 usually indicate progressive submergence of a preceding land area. This 

 interpretation, however, is permissible only so long as these deposits 

 follow a determinable stratigraphic hiatus. Exceptionally they may 

 occur during the course of otherwise uninterrupted sedimentation (see 

 figure 17 E, page 450) as when land-wash for some climatic reason is 

 temporarily increased. Again, they may be formed along the shore of a 

 restricted sea, providing of course that conditions favoring transportation 

 of land detritus obtain in the affected areas. In all such exceptional 

 cases the stratigraphic relations of the clastic deposit are peculiar in 

 that the bed wedges out in an unbroken sedimentary sequence. 



(12) Change in character of sediments indicative of diastrophic 

 movement. — Abrupt or even relatively gradual changes in kind of sedi- 

 ment, especially if the change is from limestone to some distinctly clastic 

 deposit, always suggest diastrophic activity. This may be entirely local 

 in its immediate origin and extent. As a rule, however, the change is 

 connected with some broader deformative movement which may result 

 in similar effects in many places or in dissimilar — often, indeed, directly 

 opposite — effects in widely separated regions. Obviously there is no 

 sharp line of separation between local and general movements, and as the 

 lithological and structural manifestations are very similar in both, it is 

 often very difficult to decide between them. Further, while marine 

 sandstones frequently succeed limestone, we know also that wind-blown 

 dune and beach sands, and more rarely ordinary fiuviatile and delta 

 deposits, similarly rest on limestone. And are we not certain that the 

 lithologic change is sometimes occasioned by local climatic changes hav- 

 ing no immediate connection with decided orogenic or epeirogenic move- 

 ments? Even if I were capable of doing Justice to this phase of the 

 subject, the task would be far beyond the scope of the present work. All 

 T wish to bring out here is the necessity of close investigation of all 

 lithological changes in local stratigraphic sequences, because they are 

 richly promising in the search for stratigraphic breaks. The discovery 

 of such breaks, whether previously suggested by faunal evidence or not, 

 is the most important duty of the progressive stratigrapher. There is 

 always the chance of finding soipe new, previously unrecognized stage 

 in geologic history. 



So far as the evidence in hand will permit of forming an opinion, the 



