
118 STRUCTURAL AND FIELD GEOLOGY 
formed on a sandy or muddy beach by the trickling down- 
wards of little rills during the retreat of the tide: They are 
occasionally visible on the surfaces of fine-grained sedimentary 
rocks. When they are numerous and run into each other, 
they often simulate the appearance of some kind of alge, and ~ 
have not infrequently been described as fossil sea-weeds. 
Sun-cracks (Plate XXVI.).—-Round the shores of inland 
seas and lakes, the level of which is liable to fall during the 
dry season of the year, a wide belt of gently shelving ground 
is laid bare. The same is the case in many river-valleys— 
broad flats appearing when the rivers are low. Frequently 
such exposed tracts consist of clay or mud, which, under the 
influence of the sun, becomes dry and shrinks, so that the 
surface cracks into polygonal cakes. When the wet season 
arrives, and the level of the water rises, sand may be deposited 
over the consolidated and fissured clay, and thus a cast of the 
cracks will be formed. The same action may take place on 
low, flat beaches which are exposed to a hot sun during the 
retreat of the tide. Sun-cracks are thus of common occurrence 
in many geological systems. The casts usually adhere to the 
overlying stratum, of which indeed they form a part. 
Rain-prints.—In like manner the pits made by rain on the 
surface of fine-grained deposits have occasionally been pre- 
served. The smooth bedding-planes of argillaceous sand- 
stones, shales, and mudstones are not infrequently pitted in 
this way, the casts of the pits occurring on the under surface 
of the overlying stratum. Sometimes the direction of the 
wind at the time the rain fell is shown by the inclination of 
the pits in one particular direction. In such cases there is 
occasionally the appearance of a slight ridge on one side of 
the pits, as if some of the fine sediment had been flicked out 
by the drops as they fell. 
Animal-tracks, etc.—Additional evidence of beach-con- 
ditions is obtained from tracks left by animals. Thus, tracks 
or trails of annelids, molluscs, crustaceans, etc., worm-burrows 
and castings, and the footprints of reptiles, amphibians, birds, 
‘and mammals, have been preserved, usually in fine-grained 
sedimentary strata. Now and again, also, certain puzzling 
impressions make their appearance, which have often been 
described as plant-marks. Possibly some of these may have 
