408 JOURNAL OF SCIENCE. 
largely of Globigerina-ooze. Chalk differs considerably from 
the ooze of the Atlantic in its chemical composition, but, as Mr. 
Starkie Gardner has pointed out,great chemical changes must have 
taken place in chalk since it was upheaved above the sea; the 
soluable constituents have been to a great extent removed, and 
the silica and iron have aggregated into flints and nodules. The 
palzontologicalevidenceastothe depth at which chalk was formed 
is conflicting, for while the sponges, corals, and Echinoderms are 
allied to deep sea forms, the Mollusca, according to Mr. S. P. 
Woodward and Mr. Gwyn Jeffries, indicate comparatively shal- 
low water. On this, however, Mr. Gardner remarks that “the 
dissolution from the chalk of all its Mollusca except the few 
whose shells were phosphate of lime, renders any inference from 
them relatively untrustworthy, seeing that so many of the latter 
are extinct forms, about whose habits we can know nothing.” 
Mr Murray says, “the globigerina-oozes which we get in shallow 
water resemble the chalk much more than those in deeper water, 
say over 1000 fathoms.” 
But granting that the sedimentary rocks are all shallow water 
deposits, does it necessarily follow that the continental areas 
have never formed part of the deep ocean? I think not, for the 
following reason :—The deep sea' deposits accumulate with ex- 
ceeding slowness. Shark’s teeth, belonging to extinct tertiary 
species were dredged by the “ Challenger” expedition from the 
bed of the deep sea, and they must therefore have been lying 
almost or quite on the surface of the mud. Minute granules of 
native iron, supposed to be of meteoric origin, form no incon- 
siderable part of the red clay, and this could not be the case un- 
less the deposition of other material was almost inconceivably 
slow. These deposits, therefore, must be thin and unconsoli- 
dated, and if the mid-ocean beds were to rise above sea-level, 
they would be easily removed by denudation. Now, in every 
continental area large periods of time are unrepresented by any 
rocks. This may, of course, be owing to the surface having been 
land during those periods, but it may also be owing to its having 
been at the bottom of’ a deep ocean. Consequently we can 
arrive at no definite conclusion with our present imperfect know- 
ledge. 
Mr. Darwin has pointed out that nearly all oceanic islands 
are either of volcanic origin or else coral atols, and that none of 
them shew any sedimentary rocks older than the tertiary era. 
From this he infers that no continents nor continental islands 
existed in the earlier periods where the oceans now extend ; for 
had they existed, sediments derived from them would have been 
partially upheaved by the oscillations of level which must have 
taken place. This fact, however, seems to me to point to an 
exactly opposite conclusion. We do not know why volcanic 
islands should be so common in oceanic areas ; but, whatever be 
the cause, it must have acted in the ancient oceanic areas as it 
acts now, and the absence of older sedimentary rocks on these 
islands goes to prove that they were not then in existence: else 
