CHAPTER V 



The Later Geological History of North America 



The present flora of North America, while it probably antedates 

 the arrival of man upon this continent, has not remained unchanged 

 for many thousands of years but has been constantly changing 

 and was preceded by other and ancestral floras in which the group- 

 ing of genera, the abundance of specific types within these genera, 

 and the range of the latter, was markedly different from what it 

 is today. Glimpses into these floras of the past, extending back 

 several millions of years, are obtained by piecing together the 

 evidence furnished by the fossil remains of leaves, fruits and stems 

 preserved in the rocks of the various geological formations. 



The majority of our present forest types, those belonging to 

 the class Dicotyledonae of the Angiosperm phylum (i.e., flowering 

 plants with closed seed vessels) have an ancestry extending well back 

 into the later Mesozoic or Secondary age, as the old geologists called 

 it. The conifers (pines, spruces, hemlocks, etc.) while less numer- 

 ous in existing genera and species than the hardwood deciduous 

 trees, making up for it in a measure by their individual abundance, 

 are of enormous antiquity, representatives of this group being 

 present in the rocks of the most ancient deposits in which land 

 plants have been found. 



The Mesozoic age, mentioned above, was probably the scene of 

 origin of the Angiosperms, which became abundant before its 

 close. It is often referred to as the age of conifers (more properly 

 gymnosperms) because of the abundance and variety of coniferous 

 types at that time. Faunally it is known as the age of reptiles 

 among vertebrates and the age of ammonites among invertebrates. 

 The diagram shown on page 8 serves to graphically portray the 

 various divisions into which geological time has been divided. In 

 this diagram I have given the sub-divisions of the geological periods 

 for the later ones only as those are the ones with which we are 

 especially concerned in discussing the ancestors of our forest trees. 



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