32 The American Naturalist. . [January, 
have begun their labors at depths much exceeding 20 or even 
50 fathoms. The coral wall rises upward, and it bears in its 
midst and over its surface an extensive and variegated ocean 
life. First the corals of different genera massed together in 
contiguous groups and colums, then the fan corals (gorgon- 
ias) with bryozoa, crinoids, coralline sea-weeds and sponges, 
and finally numerous sea-worms (annelids) like Serpula com- 
plete the heterogeneous assemblage with an occasional molluse 
or some sedentary crustacean. “All these things,” to quote the 
expressive description of Thompson, “living and dying, are 
constantly yielding a fine powder of lime, which sinks down 
and compacts in the spaces among their roots; and every break- 
er of the eternal surf grinds down more material and packs it 
into every hollow and crevice capable of receiving and retaining 
it.” In this dust the dying portions of the coral wall become 
entombed, and mingling with them the shattered or complete 
skeletons and remains of the associated fauna. Thus the 
whole is ready for fossilization; it is raised, or similar beds 
were raised, above the action of the ocean waves, becoming 
more and more bound together, more and more hardened and — 
more dense. The solvent action of surface waters cement it — 
together and converge through the interstices of the mass 
molecules of carbonate of lime which fill up the minute crev- 
ices, the microscopic pores, hastening the formation of a fossil- _ 
iferous limestone. In these perfectly preserved masses, heads — 
and nodules of coral are found retaining the most delicate de- 
tails of structure, and with them fragments or complete exam- 
ples of their associated guests and tenants. One of the most — 
striking illustrations of an ancient fossil coral reef is that 
offered by the Falls of the Ohio at Louisville, Ky., where — 
ledges of horizontal limestone form a low escarpment over — 
which the river plunges. The formation is Devonian, and, — 
while the softer parts of the stone have weathered away, the 
harder calcareous corals stand out in projecting groups, and 
in their commingled diversity of genera with bryozoal remains — 
and the joints, stems and heads of crinoids, forms a complete | 
reproduction of a modern coral reef. As to the condition of | 
preservation in which the corals are found, Lyell has taken ~ 
