CHAPTER XVI 



FOSSIL ARCHEGONIATES 



While the geological record is necessarily very incomplete, 

 neA'ertheless a study of the fossil forms has been of great assist- 

 ance in understanding the relationships of the existing Arclie- 

 goniates. 



Unfortunately the simpler, and presumably the older, types 

 are too delicate in structure to have left any recognisable fossil 

 remains, except in a very few cases ; and this is true also of 

 the more perishable structures, such as the gametophyte of the 

 higher forms. 



In spite of the very fragmentary nature of the fossil re- 

 mains, some of these are so complete that our knowledge, even 

 of the internal structure of some of the extinct types, is extra- 

 ordinarily accurate, and the researches of the past two decades 

 have thrown much light upon the geological history of the 

 higher Archegoniates. 



The fossil remains are of two kinds — casts and petrifac- 

 tions. The former, of course, can give information only as to 

 ■the external characters, but these impressions are in many in- 

 stances beautifully clear, and the nature of the plants unmis- 

 takable. True petrifactions are of much rarer occurrence, but 

 where they do occur, the internal structure of the petrified plant 

 can often be made out with great exactness. The infiltration 

 of mineral substances completely replaces the cell walls, and 

 thin sections of such petrifactions show most beautifully the 

 character of the tissues. Silica, calcium-carbonate, iron pyrites 

 among other substances are the causes of these petrifactions. 

 This petrifaction may take place on a large scale, as is seen in 

 the petrified forests of Arizona and California. For a full ac- 

 count of the conditions under Avhich fossils have been formed, 



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