268 



STRUCTURE COMMON TO ALL ROCKS. 



stratum, such as sandstone or grit, or by a lava-flow. In the latter case, 

 the horizontal lava-blanket gives rise to tables, whatever be the position 

 of the underlying strata. Good examples of this form are found in Illinois 

 (Fig. 233), in Tennessee (Fig. 234), in the mesas of the Plateau region 

 (Fig. 10, p. 17), and in Table Mountain, of California (Fig. 235). 



Fig. 235.— Section across Table Mountain, Tuolumne County, California: L, lava; G, gravel; 8 8, 

 slate; E, old river-bed; B', present river-bed. 



{b.) But if the horizontal strata are soft, interstratified sands and 

 clays, their erosion gives rise to fantastic castellated forms of peaks, 

 turrets, etc., such as are found in the Bad Lands of the Plains and 

 Plateau region, which are the almost unlithified deposits of the Terti- 

 ary lakes (Fig. 236). 



Fig. 236.— Mauvaises Terres, Bad Lands (after Hayden). 



2. Gently-undulating Strata.— These give rise to synclinal ridges 

 and anticlinal valleys. This is well shown in diagram (Fig. 237) and 



Fig. 237.— Diagram showing Synclinal Eidges and Anticlinal Valleys (after N5e and De Margerie). 



