THE VALUATION OF UNCONFORMITIES 201 
greatness is measured by the degree of discordance; the great uncon- 
formity being the one in which the lower series has been much more 
deformed than the upper, while the slight unconformity separates 
beds which have similar structure. 
Walcott describes the break separating the Cambrian from the 
Belt series in Montana as a “slight unconformity”! because the 
dividing line is rendered very inconspicuous by the parallelism of 
the strata above and below. But on another page he refers to the 
same interruption as a “great stratigraphic unconformity,”’* because 
it represents the loss of 3,000-4,000 feet of Algonkian strata by 
erosion. Here is an implication that an unconformity may be con- 
sidered great if there is a large “lost interval,” even if it is incon- 
spicuous because the lower and upper beds are not discordant. 
Referring to the unconformity between the horizontal beds of 
Ordovician and Carboniferous (Pennsylvanian) strata in eastern 
China, the present writer says: 
The absence of Silurian, Devonian and Lower Carboniferous series from 
Shantung indicates that the interval of erosion may have included all of those 
periods, and thus be worthy of rank as an unconformity of the first magnitude. 
It is possible, however, that sedimentation continued long after Ordovician time, 
and that the resulting rocks were subsequently removed by erosion, in all localities 
thus far examined..... 3 
In this case lapse of time is made the sole criterion of greatness, 
and is discriminated from stratigraphic break or thickness of strata 
‘missing. 
To summarize these different usages, then. an unconformity is 
sometimes called great (a) because there is prominent discordance 
of structure, (b) because a great thickness of strata is lacking, or 
(c) because the making of the unconformity involved a long lapse 
of time. It is true that these factors may all apply to any one 
unconformity, but they do not necessarily agree with each other. 
Great stratigraphic break is usually regarded as implying great lapse 
of time, and hence the two ideas are often combined in discussions 
and the expressions are used as if they were equivalent. In the study 
tC. D. Walcott, ‘‘Pre-Cambrian Fossiliferous Formations,” Bull. G. S. A., X, 211. 
2 Loc. cit., 204. 
3 Bailey Willis, Eliot Blackwelder, and R. H. Sargent, Research in China, I, Pt. 
I (1907), pp. 48-49. 
