794 J. BARRELL ~ MEASUREMFJNTS OF GEOLOGIC TIME 



by Grabau a disconformitA'.^'^ With the I'ecognition of the importance 

 and number of 8iieh planes, tlu^ word has become well established- in 

 geological literature. Many lost intervals ninst exist, however, in deposi- 

 tion which are of less dnration than those which are detected by marked 

 biotic or physical change. TJie arguments in the present article point out 

 their large aggregate time n aluc and the generality of their occurrence, 

 owing to the rhythmic character of sedimentation. They must be recog- 

 nized from the physical conditions of depositiuii, especially by sharp 

 surfaces separating bcnls, or l)y snrlden changes in the nature of the sedi- 

 mentary rhythm. 



The term disconformity has ^come to refer to a well defined hiatus of 

 larger value and essentially due to diastrophism in elevating the land or 

 depressing the ocean level, although for lack of a separate term these 

 minor breaks would also at present be included as minor disconformities. 

 The distinctness of the two classes of lost intervals and the ;emphasis 

 placed here on the class of smaller pauses, due largely to oscillations in 

 the intensity of climatic factors, seem to justify the erection of a new 

 term and the restriction of the class of disconformities to those breaks 

 wliich have a large enough value to be recognized by change in fossils 

 or marked contrasi in sedimentation. A suitable name appears to be 

 dicistem, the word meaning a space or interval. It was formerly used 

 in music as signifying an interval. The Latin form, diastema, is now 

 used in zoology with the special meaning of a vacant space, or gap, be- 

 tween teeth in a jaw. 



A disconformity marks a period of time which is represented in some 

 other region by a deposit of formation value. A diastem is a break 

 represented in other regions, often within the same formation, by a bed 

 or series of beds. A disconformity is theoretically traceable over a broad 

 area. Foi- deposits of epeiric seas it means ordinarily a movement suf- 

 ficient to more or less completely drain the sea-water from the continent. 

 A diastem does not imply the draining of a shallow sea, but rather a 

 shorter or longer downward oscillation of wave-base. The two classes 

 of breaks, although typically distinct, must grade into each other. The 

 assignment of a break to one or the other icategory must not depend on 

 a doubtfully assigned cause, but must rest on the observable field evidence. 

 Therefore the discrimination should rest for disconformities on breadth 

 of occurrence and faunal or floral change, for diastems on breaks in con- 

 tinuity of lesser areal impoi'tance, of greater number, and not character- 

 ized b}^ permanent faunal or floral change. The presence of diastems 



'3 A. W. Graba-u : Priuciples of Stratigraphy, 1913, pp. 821-826. 



