84 



LITHOLOGICAL GEOLOGY. 



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. 



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

 ripples on a sand-beach. 



(2.) Wave-marks. — Faint outlinings on a sandstone layer, like the 

 outline left by a wave along the limit where it dies out upon a beach, 

 marking the outline of a very thin deposit of sand. 



(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. 



Fig. 64. 



Fig. 65. 



(4.) Mud-cracks (Figs. 64 and 65). — Cracks intersecting very ir- 

 regularly the surface or a portion of a layer, and formed by the dry- 

 ing 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 having been solidified 

 against either wall of the crack until the two sides met at the centre 

 and became more or less perfectly united. Specimens of rock thus 

 honeycombed are sometimes called septaria (from septum, partition), 

 but the term is little used in science. 



(5.) Rain-prints (Fig. 66). — Rounded pits or depressions, made 

 by drops of rain on a surface of clay or half-dry mud. On a reversed 

 layer, the impressions appear raised instead of depressed, being casts 

 made in the pits which the rain had formed. 



