94 



STRUCTURAL GEOLOGY. 



structure is produced. It characterizes part of the " Pictured Rocks " of 

 the south shore of Lake Superior (Foster and Whituey's Report, fronr 

 Avhich the above figure was taken), and shows that the beds were not made 

 in deep waters, but above tire sea level by the drifting winds, like the drift- 

 sand ridges of a windward coast. 



(12) The mud-cracks made over a drying mud-flat are often preserved in 

 the rocks (Figs. 64 and 65), and prove the mud-flat origin of the bed. 

 Such cracks are necessarily shallow, as they are limited by the depth of the 

 mud. The cracks become filled by the sediment after a return of the waters, 



64. 



65. 



Mud-cracks. D. '49. 



and into this filling a cementing solution may pass from above. If the solu- 

 tion is siliceous, the filling becomes harder than the rock either side, so that 

 when worn, the surface is one of prominent intersecting ridgelets, as in the 

 figures. Moreover, these ridges are generally double, the filling having 

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

 center, and became more or less perfectly united. Layers having such filled- 

 up mud-cracks are very common in stratified rocks. 



(13) RippIe-marJcs (Fig. 66), a series of wavy ridgelets, precisely like 

 the ripples on a sand-beach, are also common in many sandstones, the oldest 

 as well as the latest, and are often indications of sand-flat origin, — like the 

 sand-flats off many seashores or in bays, though not necessarily so, since 

 ripples may form over the bottom as far down as oscillation in the water 

 extends, which may be a hundred yards or more ; and they are also formed 

 by the winds over surfaces of loose sand. 



(14) Wave-marks are faint outlinings on a bed of sandstone, 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. They have the same 

 kind of significance as ripple-marks, but are surer evidence that the beds are 

 of beach origin. 



