171 
THE GEOLOGICAL MIDDLE AGE. 
cent of entirely new types to some ten per cent 
of those resembling the parents. One would like 
a fact or two on which to rest so very extraordi- 
nary a reversal of all known physiological laws of 
reproduction, but, unhappily, there is not one. 
Still more unaccountable, upon any theory of 
development according to ordinary laws of repro- 
duction, are those unique, isolated types limited 
to a single epoch, or sometimes even to a single 
period. There are some very remarkable in- 
stances of this in the Cretaceous deposits. To 
make my statement clearer, I will say a word of 
the sequence of these deposits and their division 
into periods. 
These Cretaceous beds were at first divided 
only into three sets, called the Neoeomian, or 
lower deposits, the Green-Sands, or middle depo- 
sits, and the Chalk, or upper deposits. The Neo- 
comian, the lower division, was afterwards sub- 
divided into three sets of beds, called the Lower, 
Middle, and Upper Neoeomian by some geolo- 
gists, the Valengian, Neoeomian, and Urgonian 
by others. These three periods are not only 
traced in immediate succession, one above an- 
other, in the transverse cut before described, 
across the mountain of Chaumont, near Neuf- 
chatel, but they are also traced almost on one 
level along the plain at the foot of the Jura. It 
is evident that by some disturbance of the sur- 
