CLASSIFICATION 569 



characters of the marine and the non-marine series will be separately 

 treated in the order indicated. 



Classification of Types of Overlap 

 The types of overlap of sedimentary strata may be classified as follows : 



A. Irregular or discontinuous overlap. 



B. Eegular continuous or progressive overlap. 



1. Marine. 



a. Transgressive. 



b. Eegressive. 



2. Non-marine. 



c. Muviatile. 



Irregular Overlap 



Under this term we may comprise all overlap of concordant sediment- 

 ary formations, of any type, which does not proceed regularly in a given 

 direction. All overlaps of strata due to sudden inundations rather than 

 regular invasions belong here; also overlaps due to temporary deposition 

 from any cause, as seolian sediments. Generally this kind of overlap im- 

 plies some erosion of the underlying concordant formations, thus pro- 

 ducing a disconformity. A change of method of deposition may also 

 produce this kind of overlap, as the overlap of the marine Paleozoics 

 along the Front Eange region by the non-marine Eed beds. 



Progressive Overlap 



Under this term are included the types of overlap due to a regular 

 progressive onward movement of the zones of deposition, whether the 

 direction of that onward movement is landward, as in a regularly trans- 

 gressing sea,* or seaward, as in a regularly retreating seashore, and the 

 regular progressive spreading of zones of deposition, as in a growing sub- 

 aerial fan or dry delta. The first two cases constitute the marine trans- 

 gressive and regressive; the third the non-marine fluviatile type of pro- 

 gressive overlap. The lake delta may be considered as a local phase of 

 spreading river deposits. 



Progressive Overlap in marine Series 



The subject of progressive overlap of marine strata may be conveniently 

 discussed under the following headings : 

 1. Transgressive overlap. 



* This is overlap as defined by Gelkie. Text Book, 3d ed., p. 518. 



