Reviews — Dr. J. E. Marr's Introduction to Geology. 85 



from the disintegration of the newly-formed mountains, on a plain 

 eroded between the two mountain chains. 



(h) Period of cross-folding. The area traversed by two sets of 

 folds in directions W.N.W.-E.N.E. and S.W.-N.E. ; these let down 

 portions of the surface in deep pits, bounded by circular faults and 

 sharp folds. 



(c) Second base-level. The Enon Conglomerate removed, except 

 that in the fold-basins, and the surface of the valley again reduced 

 to a double level of erosion. 



{d) Kising of the land. Deep erosion of the river channels ; 

 immense gorges cut in the floor of the plain and most of tlie loose 

 contents of the Enon Conglomerate in the fold-basins removed. 



Cross-folding is the interpretation of the two sets of mountain 

 folds, and the sinking of areas in the meshes between is contrary to 

 what would happen if the folds were produced by direct tangential 

 thrust. The direct thrust theory also has to explain how a force 

 could act at a distance when the material through which it is 

 transmitted is so heavy in proportion to its strength and has such 

 immense friction to overcome. 



The resemblance of these Baviaan's Kloof fold-basins to pits found 

 between two sets of crossing ripple-marks has suggested that certain 

 mountain folds are produced by earth-shaking waves which become 

 retarded when approaching an immovable buffer, such as masses of 

 granite anchored to the deep substructure of the earth's crust. This 

 theory is further illustrated experimentally by what happens in 

 a lead sink w^here hot and cold water is let in at one end from a tap ; 

 the disturbance produced by this gradually causes ridges to form at 

 the further end of the sink, though the lead is too pliant to allow 

 a direct thrust thus to act at a distance. 



' Shearing ' and ' fold arc ' structure may also be explained 

 on the wave theory, whereas ' block up-lift ' structure is rather 

 a problem in isostasy. 



le, IE "V I IB -VT" s. 



I. — An Introduction to Geology. By J. E. Mark, Sc.D., 

 F.R.S. 8vo ; pp. viii, 229, with 33 illustrations. (Cambridge : 

 at the University Press, 1905. Price 3s.) 



rpHAT geology flourishes as a recreative science is manifest to all 

 JL who read their Quarterly Journal, Geological Magazine, 

 or Proceedings of the Geologists' Association ; while the good work 

 that is being done and may be done by amateurs as well as 

 professionals, especially in the study of fossils, formed the text last 

 year of Dr. Smith Woodward's interesting address to the Geologists' 

 Association. Most excellent work is being achieved by those who 

 concentrate their attention on certain formations, or fossils, or 

 districts ; and it is necessary that the supply of such workers be 

 maintained. 



