1CHN0L0GY. 181 



face exposed at low- water, are spread lightly over all its little 

 inequalities, and fill up every impression that may have been 

 made on it since it was left bare by the retreating waves. On 

 the return of the tide, the fine sand filling the impressions is 

 moistened, and more wet fine sand is added to it ; and a cast 

 is thus fixed in the moulds, to be more and more firmly fixed 

 by each deposition from successive tidal waves. 



Thus may be witnessed the actual circumstances daily 

 occurring that tend to preserve footprints and other im- 

 pressions made on the sea-shore, and which have operated 

 in past time to similarly preserve the impressions then 

 made on tracts alternately exposed and covered by the tidal 

 wave. The merit of having first discerned the nature and 

 cause of the numerous small hemispheric pits and tuber- 

 cular casts in relief on the surface of certain sandstone slabs, 

 is due to John Cunningham, Esq. F.G.S., architect, of Liver- 

 pool.* Since that light was thrown on their nature, they 

 have been recognized under various modifications, as impres- 

 sions of soft rain, of the big-dropped thunder-shower, of rain 

 driven obliquely by the gale, and making impressions with 

 the side of the cup highest opposite the point whence the 

 wind blew, of frozen rain or hail, etc. Dr. Dean, in 1845, 

 after witnessing the first exposure and raising of the red 

 sand slabs, near Greenfield, Mass., U. S., writes, " They were 

 characters fresh as upon the morning when they were im- 

 pressed ; ' on that morning gentle showers watered i the earth,'" 

 etc. Whenever a stratum is proved to be a " sedimentary" 

 one — i. e. to be due to the precipitation of its constituent 

 particles from water, in which they had been previously sus- 

 pended — we have evidence of some expanse of water, — proof, 

 in fact, of the existence of that element, with all its properties 



* Communicated by Dr. Buckland to the meeting of the British Association 

 at Newcastle, 1838 ; and subsequently by Mr. Cunningham to the Geol. Soc. 

 (Proc. of the Geol. Soc. vol. iii. 1839, p. 99). 



