410 Concerning Foot-Prints. [July, 
series was formed millions of years before the other. The system 
of denudation and deposition so well illustrated at Hopewell 
Cape also furnishes a striking example of one of the most uni- 
versal and far-reaching causes for the “imperfections of the geo- 
logical record.” : 
Another way in which foot-prints are preserved for long pe- 
riods of time is by becoming filled with the fine sand and dust 
borne along by the wind, which by being drifted into the tracks 
penetrates their finest markings, and becoming covered with 
more blown sand, or by silt at high tide, retain, as already ex- 
plained, an accurate mold and cast of the foot-prints, when the 
material shall have been hardened into rock. A counterpart of 
this second method of preservation can be seen when the newly 
fallen snow is drifted along by the wind and fills each cranny 
and crevice in the pavement; the snow gradually accumulates 
above, representing the sediment sifted down over the foot- 
prints on.the shore, and sometimes becomes frozen into a solid 
mass, which when removed from the walks retains on its under 
surface an accurate cast of every line and crack on the stones be- 
neath. . 
Still a third mode in which the impressions made by the feet 
of animals may be permanently preserved is seen when they 
are filled with fine sand and silt brought down by streams dur- 
ing sudden floods. The muddy waters then spread broadly out 
over the trodden sands, and cover them with a layer of fine mud; 
or, again, such a sheet of sediment may itself receive the impres- 
sions and be covered with sand by the incoming tide. 
Layers of sand and clay when once deposited not only tend 
to become consolidated by the pressure of the mass that goes on 
forming above them, but are also penetrated by water bearmg 
silica, lime, etc., in solution, which by being deposited around 
the particles of sand cement them together so as to form a com- 
pact sandstone ; the strata of mud and clay may form at the 
same time beds of slade. Sandstones and shales being exposed 
to a high temperature or subjected to great pressure, are further 
metamorphosed and form quartzite and argillite, or the common 
slate used in the school-room. Throughout all these changes, 
however, the rocks sometimes retain the forms impressed upa 
them when they were soft sand and mud. 
The oldest tracks known — excepting, perhaps, the trails left 
by annelid-like animals on the Taconic rocks of Vermont — Were  —— 
discovered some twenty-five years since in the Potsdam sandstone a. 
