THE 



JOURNAL OF GEOLOGY 



FEBRUARY-MARCH, igio 



SUCCESSION AND RANGE OF MESOZOIC AND 

 TERTIARY FLORAS^ 



F. H. KNOWLTON 



X 



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 



I This article, which should have appeared as No. X in the series of correlation 

 papers published last year, did not reach the Journal in time to be published in its 

 proper place, in No. 6, 1909. 

 Vol. XVIII, No. 2 105 



