THE CRUST OF THE EARTH. 



between the successive footprints varies from four to six feet. The 

 shorter footsteps may be taken as the length of the step when the 

 bird walked at its ordinary pace, and the larger one when it 

 moved more swiftly. 



184. A very remarkable concomitant of some of these fossil 

 footprints is the distinct Impressions of rain-drops upon the strata. 

 Dr. Dean discovered a stratum containing more than a hundred 

 marks of the feet of birds of various species, the whole surface of 

 which was pitted by the marks produced by a heavy shower of 

 rain. Like marks were observed in Storeton quarry, near Liver- 

 pool. The impressions produced by the rain-drops were sometimes 

 perfect hemispheres, an indication of a heavy fall of rain in a vertical 

 direction, and consequently in a calm atmosphere. In other cases- 

 the impressions were irregular and oblong, as if the drops had 

 struck the surface obliquely, as when a shower is accompanied by 

 a strong wind. 



Professor Hitchcock also mentions specimens of sandstone ob- 

 tained from various parts of the United States, showing at once 

 footprints, ripple-marks, and rain-drops, the latter being elongated 

 by the direction of the wind when the shower was falling. 



These phenomena can only be explained by the fact that the 

 marks were made upon the moist sand formed on the shores of an 

 estuary or tidal river, between high and low water mark, which 

 then was allowed to dry and harden by the action of the sun and 

 air between two successive tides. The waters on the return of the 

 tide would wash up silt to cover up the impressions without im- 

 pairing their accuracy ; the two layers uniting so as to exhibit 

 when separated the one a shield, and the other a cast from it of 

 the form thus impressed. 



185. Our necessary limits, rather than the exhaustion of the 

 subject, compel us here to close these first glimpses of geology. 

 We propose, however, in a succeeding paper, to resume the sub- 

 ject, and to give a brief sketch of the History of the Earth from 

 the first formation of its solid crust to the last great act of creation 

 which called the human race and its concomitant tribes into- 

 existence. 



135 



