800 The Antericdl} Geolof/isf. November, 1896 
lies in which the phiin of discordant sedimentation represents 
essentially an old peneplain, the corresponding formation 
which is deposited in the sea area is usually a limestone. In 
fact most limestone formations may be looked upon as repre- 
senting depositions during periods when the land adjoining 
was a graded surface or plain of faint relief l3nng but little 
above sea-level. This being the case, all unconformities have 
a much greater significance than has been heretofore suspect- 
ed. 
These surfaces of unconformity and their representative 
great units of sedimentation are the only absolute datum 
planes from which the measurement of formations can be es- 
timated. Theoretically the formation is generally considered 
as a fixed and clearly defined unit; in practice it is found to 
be indefinite, ill-defined and incapable of definition in any but 
the vaguest terms. But from the datum plane of the uncon- 
formity a new sequence of strata begins, sharply and clearly 
set off from the formations below. Many, and perhaps most, 
of the sharp lines of division that once existed are now effaced 
over much of the present land surface, but in this respect the 
record is not more imperfect than any other. The longer a 
land area remains above sea-level the greater is the liability 
for the records of the earlier events to be destroyed. Over 
other districts in which sedimentation has gone on without 
material interruption during an even protracted orogenic 
movement, the line of delimitation between the various forma- 
tions may not always be clearly descernible and might not, 
with existing data, be fully recognized, but with the detailed 
mapping of the country by the various official geological sur- 
veys the materials are either already at hand or soon will 
be, for sharply defining all the planes along which the lines 
of demarkation should be drawn. These lines, when once 
made out and when once properly considered, are as far-reach- 
ing and as universal in application as those of any classifi- 
catory system probably ever can be made. Where the sequence 
of events has been continuous, lines drawn through the very 
middle of a rock suc(;ession will not be j)erfectly arbitrary, 
but will be in accordance with the history' more clearlj' record- 
>ed elsewhere. 
