CHAPTER XXIII. 
Secondary formations — Brief enumeration of the principal groups — No species 
common to the secondary and tertiary rocks — Chasm between the Eocene and 
Maestricht beds— Duration of secondary periods — Former continents placed 
•where it is now sea — Secondary fresh-water deposits why rare — Persistency of 
mineral composition why apparently greatest in older rocks — Supposed univer- 
sality of red marl formations — Secondary rocks why more consolidated — Why 
more fractured and disturbed — Secondary volcanic rocks of many different ages. 
SECONDARY FORMATIONS. 
As we have already exceeded the limits originally assigned to 
this work, it is not our intention to enter, at present, upon a 
detailed description of the formations usually called ' Secon- 
dary,' the elucidation of which might well occupy another 
volume. By e secondary,' we mean those stratified rocks older 
than the tertiary, which contain distinct organic remains, and 
which sometimes pass into the strata called i Primary,' to be 
described in our concluding chapters. 
The observations which we are about to offer have chiefly 
for their object to show that the rules of interpretation adopted 
by us for the tertiary formations, are equally applicable to the 
phenomena of the secondary series. This last has been divided 
into several groups, and we shall briefly enumerate some of 
the principal of these for the convenience of reference, with- 
out pretending to offer to the student a systematic classification, 
founded on a full comparison of fossil remains. 
PRINCIPAL SECONDARY GROUPS. {Descending Series.) 
1. Strata fro?n the chalk of Maestricht to the lower green- 
sand inclusive. 
The number of species of testacea already procured from the 
different members of this division amount to about 1000. 
The principal subdivisions are the Maestricht beds, the chalk 
with and without flints, the upper green-sand, the gault, and 
