PartrL]  SUN-CRACKS, RAIN-PRINTS. 485 
rain. The way in which the muddy bottom of a dried-up pool 
cracks into polygonal cakes when exposed to the sun may be illustrated 
abundantly among sedimentary rocks. These desiccation-cracks, or 
sun-cracks (Fig. 198), could not have been produced so long as the 
- sediment lay under water. Their existence therefore among any 
strata proves that the surface of rock on which they lie was exposed to 
the air and dried before the next layer of water-borne sediment was 
deposited upon it. 

Fig. 198.—SuN-cRACKED SuRFACE oF Mup or Muppy SaAnp. 
With these markings are not infrequently associated prints of 
rain-drops. The familiar effects of a heavy shower upon a surface 
of moist sand or mud may be witnessed among rocks even as old as 
parts of the Cambrian system. In some cases the rain-prints are 
found, to be ridged up on one side, in such a manner as to indicate 
that the rain-drops as they fell were driven aslant by the wind. The 
prominent side of the markings therefore indicates the side towards 
which the wind blew. 
Numerous proofs of shallow shore-water, and likewise of exposure 
to the air, are supplied by markings left by animals. Castings, tubular 
burrows, and trails of worms, tracks of molluscs and crustaceans, 
A 
Je 
. Bi ’ VA 
dif 
YW 
Fig. 199.—Foorprints FRoM THE Triassic SANDSTONE oF ConnecTicuT (HiTcHcock). 

fin-marks of fishes, foctprirts of reptiles, bizds, and mammals, may 
all be preserved an@ give théir evidence tegarding the physical con- 
ditions under which ‘sedimentary formations were accumulated. It 
may frequently be noticed: that such impressions-are associated with 
