Geology and Mineralogy. 



11. Geology and Mineralogy. 



1. Does the actual vegetation of the Olobe fur?iish any general 

 marks by which it could he recognized in all countries if it became 

 fossil f— This question is asked by Alph. DeCandolle in a brief ar- 

 ticle in the Archives des Sciences of Geneva for December, 1875, 

 The question is answered in the negative, as was inevitable. For, 

 as the author observes, the species of plants over the globe differ 

 so widely with difference of locality that it would be exceedingly 

 difficult, or rather, impossible, to draw the line between differences 

 in species due to local distributions, and those due to successional 

 relations. The difficulties, moreover, are greatly increased through 

 the fact, well illustrated by Dr. Gray, that the vegetation of the 

 northern hemisphere has widely changed place during even the 

 Quaternary, and also more than once in earlier time. It hence 

 follows, as DeCandolle urges, that any conclusions as to the suc- 

 cession or cotemporaneity of species in Europe could not be ex- 

 pected to be applicable to America or the other continents ; and 

 even the deposits of the several natural regions of a continent 

 would not admit of being synchronized without great doubts over 

 the conclusions. This special inference is not new to geologists ; 

 for they admit that with the best of evidence they cannot make 

 out, except very uncertainly, the equivalency of the successive 

 rocks of Europe and America. 



But while this general proposition is well sustained, other ques- 

 tions are suggested by the author which appear to demand a ref- 

 erence to a wider range of facts than his paper considers. 



Professor DeCandolle seems to regard all fossils as equally poor 

 registers of geological age with plants. It is certain that fossil 

 plants are a most unsatisfactory means of determining equivalency. 

 Mari?ie plants— in wonderful contrast with marine animals — have 

 varied little through the geological ages ; and hence if plants are 

 used at all for chronological purposes we are confined, with hardly 

 an exception, to the terrestrial species. But the terrestrial species, 

 ■'yhile much more diverse than the marine, include only a very 

 limited series of distinct types, and tloras have continued the same 

 or similar through very long ages. Besides, terrestrial species, 

 whether vegetable or animal, are more confined in their distribu- 

 tion through physical conditions than those of sal^water; and, fur- 

 ther, they are far more poorly represented in the rocks than marine 

 species. ' For these reasons, and because of the great doubts that 

 come from migrations, the geologist makes little use of fossil plants 

 exce{)t for the" purpose of characterizing in a general way the floras 

 of the grander divisions of geological time. In actual fact, geolo- 

 gists, in their subdivisions or identifications of formations, have 

 relied almost solely on evidence from fossil animals, and especially 

 marine animals ; and if fossil plants are mentioned as the charac- 

 teristics of a period or age, it has been, with rare exceptions, only 

 after the question of the period or age has been decided by means 



