1886.] Geology and Paleontoiogy. 267 
over what is now Central Europe, which constantly increased in 
magnitude. We need not believe that this gulf was formed by 
any sudden catastrophe, for there is no reason to doubt that the 
sea conquered the land by the same methods and at somewhere 
about the same rate that it encroaches now, and that therefore its 
advance over many thousands of square miles of zerra firma would 
be an exceedingly lengthened process. We cannot gauge the time 
this occupied, but we know that since the appearance of man 
Southampton water has been formed, and a tract between Alum 
bay and Studland, some fifteen miles long and five or six miles 
broad, has been swept into the sea, and several species like the 
mammoth have become extinct. The rate of the encroachment 
depends mainly on that of the subsidence and the original height 
of the land, but what has here been effected in a subsidiary area 
serves to show roughly how vast a time must have been needed 
for the chalk sea to have crept from Kent to the Crimea, and 
covered the enormous area of Europe over which its traces still 
remain. As the land subsided and became sea, blue and green 
muds were thrown down, to be succeeded in due course by the 
deeper deposits of chalk ooze. It would be physically impossible 
for chalk, supposing it to represent globigerina ooze,' to be 
1 True chalk is a pure white limestone, composed of the remains of Foraminifera, 
valves of Cytherina, excessively minute infusoria, cell prisms of In erami, sponge 
mitted to be a truly oceanic deposit, o Aile nature to Jobigiian ooze, but Mr. 
Wallace, supported by the late Dr. George Jeffreys, has lately put forward the view 
that it Its vast extent, gl cp nature, and 
while its larger organisms, mainly Echinodermata or sponges, are with some excep- 
tions, such as are now met with in abyssal depths. Mr. Wallace laid some stress on the 
difference in composition of fresh globigerina ooze and chalk, as shown by analysis ; 
but Mr, Murray has recently stated that the percentage of carbonate of lime varies from 
40 to 95 in the ooze. ison took no account of the factthat the chalk had 
segregated into crystalline masses, its manganese into dendritic markings, silice- 
te . . 
iss 
sought shallower water now in order to find an equal temperature, and | i 
zu s em a ica The blue and green 
thei the Challenger pass into globigerina ooze with an increased di and 
k ault and greensand pass into chalk in exactly the same way. 
alternative theory of Wallace, that chalk is decomposed coral mud, could not 
been advanced by a geologist, as, while the chalk contains some wel erve 
‘Solitary corals, not a reef-building coral has ever been met with either in or surround- 
Img it, nor even in any contemporaneous deposit. are Lae 
