492 
ELEMENTS OF GEOLOGY. 
dense (see Figs. 582, 583); its antiquity is such that the dis¬ 
tance of time which separated it from the Upper Cambrian pe¬ 
riod, or that of the Potsdam sandstone, may, says Sir W. Lo¬ 
gan, be equal to the time which elapsed between the Pots¬ 
dam sandstone and the nummulitic limestones of the Tertiary 
Fig. 582. 
Fig. 583. 
^ k 
Eozoon Cmiadense^ Daw. (after Carpenter). Oldest known organic body. 
Fig. 582. a. Chambers of lower tier communicating at +, and separated from adjoin¬ 
ing chambers at p by an intervening septum, traversed by passages. 5. Cham¬ 
bers of an upper tier. c. Walls of the chambers traversed by line tubules. (These 
tubules pass with uniform parallelism from the inner to the outer surface, opening 
at regular distances from each other.) d. Intermediate skeleton, composed of ho¬ 
mogeneous shell substance, traversed by stolouifarous passages (/) connecting the 
chambers of the two tiers, e. Canal system in intermediate skeleton, showing the 
arborescent saceodic prolongations. (Fig. 583 shows these bodies in a decalcified 
state.) /. Stoloniferous passages.—Fig. 583. Decalcified portion of natural rock, 
showing canal system and the several layers ; the acuteness of the planes prevents 
more than one or two parallel tiers being observed. Natural size. 
period. The Laurentian and Huronian rocks united are about 
50,000 feet in thickness, and the Lower Laurentian was dis¬ 
turbed before the newer series was deposited. We may nat¬ 
urally expect that other proofs of unconformability will here¬ 
after be detected at more than one point in so vast a succes¬ 
sion of strata. 
The mineral character of the Upper Laurentian differs, as 
we have seen, from that of the Lower, and the pebbles of 
gneiss in the Huronian conglomerates are thought to prove 
that the Laurentian strata were already in a metamorphic 
state before they were broken up to supply materials for the 
Huronian. Even if we had not discovered the Eozoon, we 
might fairly have inferred from analogy that as the quartz¬ 
ites were once beds of sand, and the gneiss and mica-schis.t 
derived from shales and argillaceous sandstones, so the calca¬ 
reous masses, from 400 to 1000 feet and more in thickness, 
were originally of organic origin. This is now generally be¬ 
lieved to have been the case with the Silurian, Devonian, Car¬ 
boniferous, Oolitic, and Cretaceous limestones and those num¬ 
mulitic rocks of tertiary date which bear the closest affinity 
to the Eozoon reefs of the Lower Laurentian. The oldest 
stratified rock in Scotland is that called by Sir R. Murchison 
e 
