10 Essay introductory to Geology. 



ancient order. A few very singular species of monocotyledoueae, and a 

 peculiar genus of coniferae appear, but as yet we find no cycadeae. 



In the next period, including the oolites and cretacea, the cycadese make 

 their appearance, and become very prevalent ; these, together with the 

 conifeiEC, whicli may be associated into a particular class, constitute, not as 

 at present, ^^- of tlie vegetable species, but more than one half of this 

 upper secondary flora. The remainder of this flora consists principally, 

 as before, of vascular cryptogamias, but of new species. The monocotyle- 

 doneffi are still rare, and there are no signs of the existence of dicoty- 

 ledonCcE, which constitute four-fifths of the actual vegetation.* 



We have lastly to proceed to the examination of the fossil Fauna and 

 Flora of the most recent or tertiary order of formations ; and here we shall 

 perceive, in every instance, a gradually increasing approximation to the 

 existing forms of nature ; this approximation becoming just apparent in 

 the lowest groupe of tertiaries, partial in the middle groupe, and complete 

 in the upper; whence they have been denominated by the author, who has 

 thrown the clearest light on the subject, 1. Eocene, (the dawn of a recent 

 state) ; 2. Meiocene, (the less recent) ; and 3. Pleiocene, (the more recent). 

 It has been ascertained that these groupes contain respectively the increas- 

 ing proportions of three, nineteen, and fifty-two per cent, of fossil species 

 of testacea,t identical with those which still actually exist. The great 

 abundance of buccinoidian univalves is peculiarly characteristic of these 

 groupes. The same remarks will apply to the encreasing number and ap- 

 proximation to recent types of the marine Crustacea. The older saurians 

 have disappeared, and are replaced by a species of crocodile, which first 

 exhibits in the articulation of its vertebra?, a type exactly agreeing with 

 the recent species. Several species of turtles likewise occur. The fish of 

 the tertiary formations approach nearest to recent fish, yet hitherto only a 

 single species exactly identical, has been discovered. In the inferior ter- 

 tiary formations, a third at least of the species belong to genera which 

 exist no longer. The sjjecies of the superior tertiary formations are re- 

 lated, for the most part, to genera now common in the tropical seas. As 

 many of the deposits appear to have been not marine, but from fresh-water 

 lakes, + we have also lacustrine species of testacea, &c. and the proximity 

 of dry continents beii g thus indicated, we may naturally expect to find, as 



• In the inferior oolite, of Yorkshire, equisetaceoc prevail. The abundance of ve- 

 getable matter is here sufficient to yield regular carbonaceous strata. 



t Corals are perhaps less generally prevalent in the tertiary than in the secondary 

 deposits, yet the middle tertiaries in Lower Styria afford great abundance, composing 

 a coralline limestone, which much resembles the coral rag of our middle oolites, 

 both very accurately representing the coral reefs which prevail in tropical seas. 



J In many instances we may observe alternating series of lacustrine and marine 

 deposits, as if the same tract had, from some oscillation of the level of its surface 

 relatively to the level of the sea, been exposed to repeated submersions and alternate 

 emersions ; but this is not the place to enter into theoretical discussions. 



