200 ELIOT BLACKWELDER 
The criteria for identifying unconformites under varying condi- 
tions have been so carefully studied and systematized in recent years 
that they are generally well understood. The average student of 
geology knows that unconformities may be identifed by means of 
basal conglomerates, by weathered zones orm the underlying forma- 
tions, by the truncation of dikes, faults, and other structures, by 
the general field-relations of the outcrops, by actually observed dis- 
cordances and irregularities at contacts, and by still other means." 
At first the events signified by an unconformity were somewhat 
indefinitely realized. That the structure indicated an episode of 
erosion and the absence of strata which existed in other places, and 
that in some cases more or less disturbance had taken place in the 
intervening time, was clearly apprehended. Geologists are now gener- 
ally agreed that an unconformity implies: (@) cessation of deposition 
(usually involving emergence, and often accompanied by deforma- 
tion of the rocks); (b) denudation (usually by subaerial processes) ; 
(c) resumption of deposition (usually attending submergence, but 
often by terrestrial processes). It is also clearly understood that 
an unconformity represents a “lost interval,’ or lapse of time which 
is otherwise unrecorded at that place. The interpretation of this lost 
interval is the chief subject of the present paper, and is the one which 
most requires analysis and a definition of factors. 
Writers of papers on stratigraphy not uncommonly state that a 
given unconformity is a great unconformity, or that another is a 
slight one; that it represents a vast lapse of time, or a minor episode 
only. The reader, however, cannot always know just what is 
meant by these expressions. From the context of such papers one 
may infer that the unconformity is considered great by one writer 
because of one feature, and by another because of a very different 
feature. A few examples will make this clear. 
Regarding the unconformity at the base of the Keweenawan series, 
Van Hise says, “ 
is not great, there is such a likeness in strike and dip of the two series 
Here apparently 
t The criteria are exhaustively treated by Van Hise in a paper on the ‘“‘Prin- 
ciples of Pre-Cambrian Geology,” U.S. Geol. Surv., 16th Annual Report, Pt. I, 1896. 
2 “A Historical Sketch of the Lake Superior Region to Cambrian Time,” Jour. 
of Geol., I (1893), 127. 
. in areas in which the unconformity... . 
2)2 
as to suggest, at first, that the two are conformable. 
