THE VALUATION OF UNCONFORMITIES 2905 
to be developed at C in Eocene times and ceased to be made at C 
in the fifth post-present epoch. Its time-value reaches its maximum 
at that place, covering ten to eleven epochs, while at the present 
Gulf coast (A) its value decreases to zero. This illustrates in a very 
simple way the principle that most unconformities gradually increase 
or decrease in time-value from place to place; that there is a waxing 
and a waning phase corresponding to recession and incursion of the 
sea or to the shifting of the sites of continental deposition. If the 
recession or invasion is very rapid, the lower or upper line of our dia- 
gram will approach the horizontal, but such changes will be matters of 
degree, not of kind. 
Actually, however, the relations are rarely as simple as this, nor 
A B Cc 
Gulf border Miocene belt 
Eocene belt 
Syne Ky One Oy ech th Gig tea pes cons 
Post - present 
E pechs 
w 
Present 
Pleistocene [.*.°. 
Pliocene 
Miocene 
Ce ee ee ee 
‘Oligocene 
Eoce ne 
Fig. 2.—Diagram of a simple unconformity. The dotted area represents sedi- 
mentation, the blank space erosion. The horizontal lines denote equal time. 
are the advances and retreats as regular. In the case just given it 
will also be observed that the stratigraphic hiatus varies almost 
directly with the time-value, a phenomenon which is somewhat 
common among unconformities, but is by no means the rule. 
A case of average complexity may be formulated by imagining the 
middle Atlantic seaboard of the United States to be gradually sub- 
merged (Fig. 3). In an early stage of the encroachment of the sea 
modern sediments would be laid down upon Pleistocene sand and 
clay in the Chesapeake region (A); farther west younger beds would 
rest upon Miocene (B)—much as in the Texas example. Still later 
beds, however, would overspread the Piedmont belt with its ancient 
crystalline rocks. The discordance would suddenly change from 
slight to very great (C); and likewise the hiatus, which was equivalent 
to Tertiary-Modern on the coastal plain, would quickly expand to 
