94 



LITHOLOGICAL GEOLOGY. 



over it by the drifting winds at a new inclination, and this 

 violent removal and replacement often and variously repeated. 

 Fig. 61/ representing this mode of structure, is from Foster & 

 Whitney's Eeport on the Sandstone Eocks of Lake* Superior. 

 Fig. 61 e is also from the same work. 



101. Besides these kinds of structure, there are markings in the 

 strata which are of related origin, — viz. : ripple-marks, wave-marks, 

 rill-marks, mud-cracks, and rain-drop impressions. 



Fig. 62. 



Fig. 63. 



(1.) Ripple-marks (fig. 62). — A series of wavy ridgelets, like the 

 ripples on a sand-beach. 



(2.) Wave-marks. — Faint ou timings, of curved form, on a sand- 

 stone layer, like the outline left by a wave along the limit where 

 it dies out upon a beach. 



(3.) Rill-marks (fig. 63). — Little furrows made by the rills that flow 

 down a beach after the retreating wave or tide, and which become 

 apparent especially where a pebble or shell lies, the rising of the 

 water upon the pebble causing a little plunge over it and a slight 

 gullying of the surface for a short distance. 



(4.) Mud-cracks (figs. 64 and 65). — Cracks intersecting very irregu- 

 larly the surface or a portion of a layer, and formed by the drying 

 of the material of the rock when it was in the state of mud, just 

 as a mud-flat left exposed to the drying sun now cracks. The 

 original cracks are usually filled with a material harder than the 

 rock, so that when it becomes worn the surface has a honeycomb 

 appearance, from the prominence of the intersecting ridgelets, as 

 in fig. 65. Moreover, these ridges are generally double, the filling 



