THE PALEONTOLOGIC RECORD 23 



A word may be said as to the presence of land plants in marine de- 

 posits. That the trunks of trees may float for a considerable time and 

 to great distances is undeniably possible, but unfortunately the study 

 of fossil wood has not yet reached that degree of refinement in most 

 cases that will permit of its general use, and reliance in identification 

 must be placed largely in foliar and reproductive organs. The delicate 

 fronds of ferns, leaf-clad branchlets of conifers and the leaves of seed- 

 bearing plants are incapable of long withstanding the immersion and 

 wave action of salt waters. In my judgment, therefore, the presence 

 of fronds, leaves and similar organs in marine deposits argues very 

 near-by land. 



The only other point I shall consider is the bearing of plants on the 

 interpretation of climate. Since it is generally acknowledged that 

 plants furnish the most reliable data for this phase of the subject, an 

 inquiry as to the kinds of plants that have been found most valuable 

 in this connection may be of interest. Obviously our interpretation of 

 the probable conditions under which the plants of past geological ages 

 grew, must be on a basis of a knowledge of present conditions found 

 to obtain for similar or closely related groups. That we may occasion- 

 ally err in this is possible, especially if reliance is based on too few 

 forms, but when all the various elements of a flora are considered, the 

 results are thought to be within a close approximation of the truth. 

 Thus, since Artocarpus the bread-fruit tree only grows at the pres- 

 ent day within 20 of the equator, it follows that when Artocarpus is 

 found fossil in Greenland, 72 N., the conditions at the time it 

 flourished there must have been tropical or subtropical, and this con- 

 clusion is confirmed by the tree ferns and cycads associated with it. 

 Palms can not flourish with a temperature below 40; a fossil flora, 

 rich in palms of well-defined types, could hardly have grown under 

 very much cooler conditions. Tree-ferns are practically confined to 

 within 30 of the equator and a temperature of approximately 60. A 

 fossil flora, such, for example, as the Triassic of Virginia, that contains 

 numbers of tree-ferns, must have grown under tropical or subtropical 

 conditions. A fossil flora rich in types, the living representatives of 

 which can withstand a temperature of 40 to 60, or even lower, 

 must have been at least cool-temperate. Cycads are now found only 

 within 30 of the tropics; a rich cycad flora argues then for a tropical 

 or at least a subtropical climate. 



Examples of this kind could be multiplied almost indefinitely. In 

 interpreting geological climate selection is made so far as possible of 

 the plants or groups of plants, that are confined at the present day 

 within relatively narrow limits of temperature, be this high, medium 

 or low. 



