THE VALUATION OF UNCONFORMITIES 207 
reduced to insignificance (although the eroded surface itself would be 
very uneven). Stratigraphic hiatus decreases to a value of Pennsyl- 
vanian-Modern. ‘Time-value, however, suffers no corresponding 
change from the previous locality. 
It appears then that the three factors, discordance, hiatus, and 
time-value, may vary suddenly and largely, but that they do not 
necessarily vary in the same degree, or even in the same phase. 
Time-value may decrease while hiatus increases. Discordance 
may occasionally become more pronounced as the lost record becomes 
smaller. 
These considerations lead to the conception that unconformities 
are constantly fluctuating features of the stratigraphic record. It 
is also plain that the time-value of an unconformity can be determined 
only through the study of the geologic history of a considerable region, 
while discordance and hiatus can often be ascertained from individual 
sections. 
The hypothetical case just considered, of an unconformity in 
eastern United States, fails, however, to give a complete picture of that 
unconformity since the assumed case represents only one withdrawal 
and advance of the sea. Asa matter of fact we are to think of such a 
region as the Piedmont belt, or, let us say for the present example, the 
Canadian shield of ancient rocks, as being a land nucleus of varying 
dimensions, now expanding by withdrawal of the epicontinental sea, 
until it includes a continent, and then contracting to a mere island, 
perhaps smaller than Greenland, as the sea overspreads its shelving 
surface. These advances and retreats have taken place, not once, 
but many times, and the result is quite as many unconformities, 
all of which blend landwards into one far greater unconfornity which 
records the continuous land condition of the central nucleus. 
This is illustrated by Fig. 4, which shows an interpretation’ of 
the submergences and emergences of a part of central United States 
since pre-Cambrian times, with the corresponding sedimentary series 
and unconformities. Vertical spaces represent periods of time, while 
on the horizontal line are scaled off certain broad provinces stretch- 
t In this and other examples used in the present discussion the facts are but 
imperfectly known, nor does the writer profess to have made an exhaustive study of 
even those which are available. His interpretation of the history is not in any way 
essential, but will serve the purpose of illustrating the general principles. 
