28 
POPIJLAE SCIENCE KEVIEW. 
For a most able resume of the fresh facts established by the 
expedition on which the Challenger was despatched the writer 
is indebted to a lecture delivered by Professor Huxley at the 
Royal Institution.* The existence of a fine clay in the deeper 
parts of the Mediterranean had been already known, but no 
one suspected the existence of deposits of barren clay in the 
middle of the great calcareo-silicious zone in the open ocean. 
The dredgings of the Challenger have further established that 
certain ocean- valleys contain thick deposits of finely divided 
“ red clay,” the nature of which has been already described, 
e. g. between Teneriffe and St. Thomas, at the depth of about 
3,000 feet. If it be, as Professor Wyville Thomson suggests, 
that the deposits of “ red clay ” represent remains of myriads 
of marine organisms, not only silicious and calcareous, but also 
argillaceous deposits may be formed by long-continued vital 
agency. Further, it follows that rocks of almost any mineral 
composition may be indirectly or directly generated by living 
organisms. It is certain that — 
(а) Beyond certain depths the calcareous organisms which 
must fall over the area covered by the ocean disappear, and 
their place is taken by a fine red clay. 
(б) When ordinary Globigerina ooze has its calcareous matter 
removed, a residue of fine red clay remains. 
Supposing that the earth were covered uniformly with water 
to a depth of 2,000 fathoms (or about two miles), the merely 
tidal and current movements already existing would be insuffi- 
cient to cause any important degradation of the crust, so that 
there would scarcely be any sedimentary deposits. Let, then, 
Diatoms, Foraminifera , Radiolaria , and Sponges be intro- 
duced ; there would then be formed the silicious Pole-caps, and 
the intermediate zone, which might accumulate till beds of 
chalk were formed many thousand feet in thickness. The 
chalk might next be converted into limestone, and so all traces 
of its origin would be effaced. Next, let parts of. the area be 
depressed to 1,800 feet and other parts be raised to within 1,000 
feet; then, judging by what we now know, the former might 
be replaced by red clay and the latter by greensand. f The 
clay might be metamorphosed into shales, slates, &c., and the 
greensand into minerals into the composition of which silica, 
alumina, iron, and potash enter ; so that the imaginary world 
would be covered with rocks all due to an organic origin, but of 
* On the Recent Work of the Challenger Expedition , and its Bearings on 
Geological Problems . Friday evening, Jan. 29, 1875 . — (( Proc. Royal Inst.” 
vol. vii. part 5 (No. 62), April 1875. 
t A modern greensand in course of formation was found by Professor 
Wyville Thomson in the region of the Agulhas current. 
