84 OUT11M ^ OF FIELD-GEOLOGY PARTI 



Confidently identify former land-surfaros, lake-floors, 

 and sea-bottoms, (i) Land-surfaces are revealed to us 

 l>y layers of terrestrial vegetation resting upon what must 

 once have been soil and still contains the roots of the 

 plants that grew upon it. Stumps of trees in their 

 position of growth, with, it may be, their fruits and leaves 

 lying around, and even an occasional wing - case of a 

 beetle, or the remains of a lizard or land-snail, furnish 

 unimpeachable proof that the localities where they occur 

 were once tree -covered tracts of ground. Hence the 



FIG. 17. Rain-prints on sandstone. 



occurrence of such a terrestrial layer in a group of strata 

 proves that during their deposition a pause ensued, and 

 their site became land. Traces of ancient shores, or at 

 least of shallow water, are often preserved in ripple-marked 

 surfaces of sandstones (Fig. 16) on which the trails or 

 burrows of annelides may now and then be observed. If 

 rain-prints (Fig. 17) are associated with rippled surfaces, 

 they conclusively prove the sediment to have accumulated 

 on a shore. Further evidence of the occasional exposure 

 of the deposits to air and sun is yielded by the desic- 

 cation-cracks so commonly found among sandstones 



