182 PALEONTOLOGY. 



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.* Many of the micacious flags of the same forma- 

 tion are covered with ripple, or current marks. They shew 

 borings of worms, and a trace of a trilobite (Paloecypyge) 

 nearly allied to the Bihelocephalus — the oldest known trilo- 

 bite 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 ;f 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. 



Protichnites septem-notatus (fig. 82). 



The subject so named consists of a series of well-denned 

 impressions, continued in regular succession along an extent 

 of 4 feet ; and traceable with an inferior degree of definition 

 along a further extent of upwards of 2 feet. 



In the extent of 4 feet there are thirty successive groups 

 of footprints on each side of a median furrow, which is alter- 

 nately deep and shallow along pretty regular spaces of about 

 2i inches in extent. The number of prints is not the same 

 in each group ; where they are best marked, as in fig. 82, 1 L, 

 we see 3 prints in one group, a, a\ a!\ 2 prints in the next, b, 



* Salter, Quar. Jour, of the Geol. Soc, vol. xii., 1856, p. 250, pi. iv., fig. 4. 

 f lb., vol. viii., p. 214, 1852. 



