ICHNOLOGY 157 



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. Deane, 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 the earth,'" 

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

 one — i. c, 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 

 of condensation by cold, and expansion and vaporization by 

 heat and exposure. Evaporation makes the raw material of 

 rain. No wonder, then, that impressions of rain-drops should 

 be seen on the oldest sedimentary rocks. Conditions are co- 

 ordinated in meteoric as in organic phenomena ; one being- 

 given, the rest may be deduced. 



The oldest rocks in which rain-drop impressions have 

 been observed are those of the Cambrian age at Longmynd, 

 Wales, described and figured by Mr. Salter.f Many of the 

 micaceous flags of the same formation are covered with ripple, 

 or current marks. They show borings of worms, and a trace 

 of a trilobite (Palccojwjge) nearly allied to the Dikeloc&phalus 

 — the oldest known trilobite of America (Lower Silurian or 

 Cambrian at St. Croix, Minnesota). 



It is in " Potsdam sandstones" of the same geological anti- 

 quity that the impressions have been discovered which the 



* 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.) 



f Quar. Jour, of the Geol. Soc, vol. xii., 185G, p. 250, pi. iv., fig. 4. 



