204 ELIOT BLACKWELDER 
indicate a period of erosion of long or short duration, not to be 
estimated by the “lost record.” 
At this point it will be advisable to consider how these three factors 
may be determined with reference to a given unconformity. The 
degree of discordance may often be observed directly in sections, or 
may be inferred from observations of strike and dip. The strati- 
graphic hiatus may be discovered by correlating the beds above and 
below with a standard section of reference (supposing that such a 
section has been established), and thus determining what formations 
are lacking. ‘These are matters of common knowledge and need not 
be dwelt upon here. The time-value, however, is not so easily ascer- 
tained, since observations on one section, or even on several adjacent 
sections, are not sufficient to bring out the facts. Let us start with 
the generally understood principle, ably presented in recent years. 
by Chamberlin and Salisbury," that all uncenformities are presumably 
limited in extent; when traced in any direction they are eventually 
lost in a conformable sequence of strata. ‘Thus the present eroded 
surface of North America—a future unconformity—merges into the 
continuous sediments of the seas about its borders. But some parts. 
of this land mass have been out of water much longer than others, 
and so the unconformity which is to be will have a different time- 
value in different places. For example, if the sea-level should 
rise steadily the sea would encroach upon the land. With it would 
come the sedimentation for which it furnishes the conditions. In 
the southern Great Plains these modern sediments would lie first. 
upon the Quaternary beds of the Gulf border. As the sea has been 
only recently excluded from this strip, the time-value of the intervening 
unconformity would be small—probably a fraction of the Quaternary 
period. Further slow advance of the sea would allow somewhat 
later deposits to be laid over a surface which seems to have been land 
since Miocene times. Continued encroachment would eventually allow 
deposits of still later age to be spread upon land which has been eroded 
presumably since the Eocene epoch. Here evidently the time-value 
is greater than in either the first or the second locality. The events 
and time-intervals are expressed graphically in the accompanying dia- 
gram (Fig. 2). This particular unconformity, it will be observed, began 
tT. C. Chamberlin and R. D. Salisbury, Geology (1906), II, 222-24. 
