CLASSIFICATION OF RIVERS 133 



further laking; or interlacustrine, spilling over from one lake into 

 another. The St. Lawrence River is a good example of the former 

 type, and the Niagara of the latter. Overflow streams are charac- 

 terized by little or no sediment, this being all left behind in the 

 lake. They may, however, pick up sediment on the way from their 

 bed and banks, or from tributaries, and so obtain tools for erosion. 

 Niagara is a good example of such a stream, free from sediment in 

 its upper part and accomplishing its erosion work by undermining 

 at the cataract and drilling on the floor. 



IV. Glacial Streams. These have their water supplied by the 

 melting ice mass, which also furnishes detrital material. Hence 

 these streams are distinct from all others. Streams originating on 

 the ice, i. e., superglacial streams, have the consequent habit, and 

 might be classed as an extreme type of that group. But sub- 

 glacial streams are unlike all others. Their peculiar position under 

 the ice cover and subject to hydrostatic pressure, often forces them 

 to flow across obstacles no normal river could surmount, or actually 

 to flow for a part of their course uphill. Such abnormal courses 

 of ancient subglacial streams are indicated by the position of the 

 eskers which they have built. In many characters, subglacial 

 streams approach subterranean streams. 



V. Subterranean Streams. These are often of great extent and 

 diversity, following enlarged joints in limestone strata, and forming 

 caverns along their paths, which are subsequently more or less 

 filled up again by stalactic deposits. L^nderground drainage may 

 develop to the complete absorption of surface drainage, giving the 

 peculiar Karst type of landscape so characteristic of the west side 

 of the Balkan peninsula, and also seen in the Barren lands of 

 Kentucky and Tennessee, in the plateau of the Cevennes in southern 

 France, in the German Jura, and in many other regions. (Cvijic-5 ; 

 Neumayr-27 -.jOO-^IO.) 



Polygene Rivers. 



Under this heading may be grouped the three types denomi- 

 nated by Davis: i. compound rivers, 2. composite rivers, and 3. 

 complex rivers. (Davis-S:/^?; g:i02.) Davis defines a compound 

 river as one "which is of different ages in its different parts," and 

 cites as examples "certain rivers of North Carolina which have old 

 headwaters rising in the mountains and young lower courses tra- 

 versing the coastal plain." These are the extended consequents of 

 our classification. The term should be made to cover streams 

 which by capture have enlarged their drainage basin at the expense 



