There is a third condition under which impressions, and casts of impressions, on a sandy beach may be preserved. On a dry windy day clouds of fine sand are drifted along the surface 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 conditions and the cir cumstances daily occurring that tend to preserve footprints and other impressions 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. 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. 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 watet—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.t Many of the micaceous flags of the same formation are covered with Tipple, or current marks. They show borings of worms, and a trace of a trilobite (Pakeerpyge) nearly allied to the Dikelocephalus —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 writer has interpreted to be those of a large entomostracous Crustacean ;* in evidence of which the following sample, appli cable to a single species, may be given, in illustration of the ichnologist's mode of work.