CHAPTER X 

 SUCCESSION AND RANGE OF MESOZOIC AND TERTIARY FLORAS' 



F. H. KNOWLTON 



It is of course a truism to say that the transition from the Paleozoic 

 to the Mesozoic is not, as was once supposed, an abrupt or catas- 

 trophic change, but was brought about so gradually that in many 

 parts of the world it is often difficult, if not indeed impossible, to 

 draw any sharp lines. Not only are the rocks lithologically similar, 

 but a certain percentage of life-forms persisted from the one to the 

 other, yet when each system is considered in its entirety there are 

 apparent abundant lithologic and strongly marked biologic differ- 

 ences. It is my purpose to speak briefly of the floras, first of the 

 Mesozoic and later of the Tertiary. 



Triassic. — Rocks of Triassic age are known in many parts of the 

 world and indicate two types of deposition, a fresh-water, marsh, or 

 lagoon phase, and a marine phase. The former is only, or largely, 

 that which has afforded a flora. The known plants of the Trias are 

 relatively few in number. In North America we have less than 

 150 species, and the entire Triassic flora probably does not exceed 

 300 or 400 forms. Owing to considerations, physical and otherwise, 

 concerning which there is not complete agreement, the lower portions 

 of the Trias afford but scanty remains, and it is not until we come 

 to the upper portion, or Rhaetic, that it can really be dignified as a 

 flora. Our North American Triassic flora is believed to belong 

 largely to this portion. Triassic plants have been doubtfully reported 

 from Prince Edward Island, but they are so obviously of Permian 

 types that they may be disregarded. The principal areas are in 

 North Carolina, Virginia, and Pennsylvania, with relatively few in 

 Maryland, New Jersey, Connecticut, and Massachusetts. In the 

 west we have a doubtful plant or two from Wyoming, a considerable 

 number from northern New Mexico, the extensive fossil forests of 



I Published by permission of the Director of the U. S. Geological Survey. 



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