490 



GEOLOGY. 



tow and other currents passing over pebbles^ shells, etc. (Figs. 325 and 

 326). 



Sediments are sometimes exposed between tides, or under other 

 circumstances, for periods long enough to permit drying and cracking 

 at the surface. On the return of the waters, the cracks may be filled 

 and permanently preserved. These are known as sun-cracks or mud- 

 cracks (Figs. 328 and 369). They chiefly affect shales, but are occa- 

 sionally seen in limestones and fine-grained sandstones. During the 

 exposure of the sediments a shower may pass and raindrop impressions 

 (Fig. 370) be made which are subsequently filled by fine sediment and 

 preserved. The size and depth of rain-drop impressions give some hint 

 as to the meteorological conditions of far-off ages. Wave-marks, which 

 consist of the faint hne-ridges developed on a sandy beach at the hmit 

 of the incoming wave, are sometimes preserved and may be seen occa- 

 sionally on layers of rock deposited millions of years ago. 



^ Fig. 370. — Rain-drop impressions. (Brigham.) 



I Concretionary structure. — Various sedimentary formations contain 

 nodules or irregularly shaped masses of mineral matter unhke the rock 

 in which they occur. When these nodules consist of matter aggregated 

 about some center, they are called concretions. They are common in 

 sedimentary rocks, and here it may sometimes be seen that the aggre- 

 gation has taken place about a shell, a leaf, or some other organic relic. 

 The nuclei are, however, not always organic. The material of the con- 



