184 



STRUCTURAL GEOLOGY. 



quence of the manner in which they were formed. There- 

 fore, when we find them in other positions and at other 

 levels, we conclude that they have come so by subsequent 

 change. 



We must not imagine, however, that the planes between 

 the strata were ever absolutely horizontal. Strata must 

 not be likened to continuous, even sheets, but rather to 

 extensive cakes, thickest in the middle and thinning on 

 the margins and there interlapping with other strata or 

 cakes (Fig 94). Coarse materials, like sandstones and 



FIG. 94. Diagram showing thinning out of beds : a, sandstones and conglomerates; 

 6, limestones. 



grits, are more local, and thin out more rapidly, while fine 

 materials, like clays, are often very widely continuous. 

 This thinning out of strata, however, does not interfere 

 seriously with their appearance of evenness at any point 

 of observation. 



Another more important apparent exception to original 

 horizontality is what is called cross-lamination or false- 

 bedding (Fig. 95). These are liable to be mistaken for 



95. Section on Mississippi Central Railroad at Oxford (after Hilgard) : oblique 

 lamination. 



