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 ' secondary,' we mean those stratified rocks older 

 than the tertiary, which contain distinct organic remains, and 

 which sometimes pass into the strata called ( Primary,' to he 

 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 from 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 



