442 SKETCH OF PALEOBOTANY. 



Before entering upon a general survey of the development of plant life 

 as shown in this merely numerical exhibit, it will be necessary to refer 

 the reader to three diagrams (plates LYl, LVII, LVIII), which have 

 been prepared with a view to rendering the principal facts embraced 

 in the table more readily intelligible, and then to discuss each of the 

 diagrams separately, keeping the numerical data constantly in view. 

 For the execution of these diagrams I am indebted to Ensign Everett 

 Hayden, United States Navy, on duty at the National Museum in the 

 Department of Fossil Plants, who has not only plotted and drawn them, 

 but has aided me greatly in selecting from among the many possible 

 modes of graphic illustration the ones which, as I believe, most success- 

 fully serve this purpose. 



In all the diagrams an effort is made, of course in an approximate 

 and very rude manner, to indicate time-measures in terms of thickness 

 of strata, this being, however imperfect, certainly the only standard 

 attainable. In a lecture delivered at the National Museum on Feb- 

 ruary 24, 1883, on Plant Life of the Globe, past and present, enlarged 

 diagrams having a similar object to those introduced here were used 

 for illustration. The data then obtainable for their preparation were 

 very defective, and the time-measures were taken from Dana's " Man- 

 ual of Geology." Those who may remember them, from notes taken or 

 otherwise, will observe that in this latter respect the accompanying dia- 

 grams differ widely from the ones presented on that occasion. Upon 

 investigation it appears that the views of geologists generally have 

 changed materially since the appearance of the last edition of that 

 work, and recent observations have tended to show that the thickness 

 formerly assigned to Mesozoic, and especially to Tertiary, strata was 

 much too small in proportion to that assigned to Paleozoic, and especi- 

 ally to Silurian strata. After consultation upon this subject with the 

 Hon. J. W. Powell, Director of the Survey, it was decided that nearly 

 equal vertical space might be given to each of the following formations, 

 or groups : 1, Cambrian ; 2, Silurian ; 3, Devonian ; 4, Permo-Carboni- 

 ferous ; 5, Jura-Trias ; 6, Cretaceous ; 7, Eocene ; 8, Mio-Pliocene. These 

 have accordingly been taken as furnishing the scale of time equivalents, 

 and all the diagrams have been drawn to this scale. 



The development of vegetable life through geologic time may be dis- 

 cussed from threesomewhat distinct points of view. We may,in the first 

 place, consider each of the principal types of vegetation at each of the 

 geologic periods in which it occurs solely with reference to its relative 

 importance in the combined flora of that epoch. This is undoubtedly 

 the most important point of view from which the subject can be contem 

 plated, and has accordingly been considered first. It is clear that the 

 data for this must consist, not in the actual number of species at each 

 horizon, but in the proportion, or percentage, which this number 

 forms of the total number found at such horizon. Diagram No. I is 

 therefore, based upon these percentages as given in the foregoing table. 



