THE VALUATION OF UNCONFORMITIES 
ELIOT BLACKWELDER 
University of Wisconsin 
The utility of unconformities in geology has long heen recognized, 
and the historical significance of such structures is becoming more 
and more clearly understood. In the early days of geologic science 
only the more clearly visible unconformities involving discordance cf 
strata were identified as such. Later it was shown that an irregular 
eroded surface between parallel beds may imply much the same 
conditions and events as the more conspicuous unconformities, except 
that deformation of the older rocks is not involved. The distinction 
was clearly made by Irving, in his admirable paper on the correlation 
of unfossilifercus rocks; the breaks accentuated by discordance were 
called “true unconformities,’’ while those in parallel strata were 
stvled “erosion intervals.”’? It was felt that the very word “uncon- 
formity”’ involved the idea of angular discordance between the beds 
above and below, and hence that any interruption between parallel 
beds must go by another name. In spite of this, however, the scope 
of the term unconformity has been gradually extended so that we 
find Le Conte, writing in 1890, combining breaks between discordant 
beds and eroded surfaces in parallel strata as merely two varicties 
of unconformity.? This usage is the one now generally followed 
by geologists (although the phrase erosion interval is still current), 
and it is in this sense that the term will be employed in the ensuing 
pages. 
We now have several types of unconformities clearly distinguished: 
(a) eroded surface separating parallel strata; (b) contact between 
rocks of wholly unlike origin (for example, sandstone resting upon 
granite); and (c) angular discordance of beds with or without differ- 
ence in lithologic character. 
tR. D. Irving, U. S. Geol. Surv., Ann. Rep., VII, 1886, pp. 392, 393. 
2 Jos. Le Conte, Elements of Geology, 3d ed., 1893, p. 180. 
3 The phrase “‘eruptive unconformity,’”’ recently used to describe broad intrusive 
contacts of granite with older schists, is here excluded, on the ground that even if such 
structures are unconformities in any sense, they differ from erosional unconformities 
so fundamentally that the two cannot well be discussed together. 
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