76 M. Brongniart on tlie different Floras which 



Acrogens^ which is at the same time that of the anomalous 

 Gymnosperms, Sigillariese, Noeggerathiese and Asterophylhtese. 



1. Carhoniferous Period. 



This long period begins with the appearance of the first ter- 

 restx'ial vegetables deposited in certain layers of the transition 

 formations, and extends to the new red sandstone which covers 

 the coal formation ; in fact, all through this period there is no 

 important difference between the forms of the plants ; they are 

 of the same families, the same genera, and often the same species ; 

 and in the existing state of our knowledge on this subject, a flora 

 of the plants of the transition formations would not differ more 

 from that of a true coal formation, than the floras of different 

 strata of one single coal basin, or those of different, closely 

 contiguous coal basins, do from each other. 



I will besides call attention to the fact, that the real epoch of 

 several of the formations considered as transitional, which con- 

 tain carboniferous layers with impression of plants, is often badly 

 determined, and remains an object of doubt and discussion for 

 geologists ; that several are perhaps nothing but true coal forma- 

 tions accompanied by rocks modified by metamorphic phseno- 

 mena, and that in so far as these deposits have not been referred 

 with certainty to the formations clearly defined under the names 

 of Devonian, Silurian or Cambrian rocks, the specific comparison 

 of their fossil vegetables with those of the coal formations will 

 furnish no useful results. 



The only coal strata considered by many distinguished geo- 

 logists as more ancient than the ordinary coal formations, which 

 are very rich in fossil plants, are those of the borders of the 

 Lower Loire, between Angers and Nantes ; now the impressions 

 which they contain belong to all the genera of the ordinary coal 

 formations, without exception, and do not furnish, collectively, 

 any character by which to distinguish them from these. 



I may add, that observations made quite recently upon a car- 

 boniferous rock, — very ancient, for it is covered by strata con- 

 taining fossil animals characteristic of the Silurian formation, — 

 confirm this opinion as to the extension of the coal vegetation 

 up to the origin of the transition rocks ; in fact, in a memoir 

 by Mr. Sharpe on the Geology of the Environs of Oporto, I 

 find that tolerably thick and numerous layers of coal which 

 are covered by schists with trilobites, orthides, orthoceratites, 

 graptolites, &c., contain a few impressions of plants, and these 

 impressions, all Ferns, although rather imperfect, appear, ac- 

 cording to Mr. Bunbury, identical or extremely near to well- 

 known species of the ordinary coal formations. These are Pe- 

 copteris cyathea and muricata, and Neuropteris tenuifolia. 



