110 J. W. DAWSON — SOME RECENT DISCUSSIONS IN GEOLOGY. 



liriodendron, sassafras, platanus, sequoia and salisburia, and especially 

 in the case of all those of which seed or fruit have been preserved ; but 

 even when the naming is inaccurate or when the number of species has 

 been unduly multiplied, the deductions as to climate may hold good, 

 though not perhaps to the extent of enabling us to fix a definite thermo- 

 metric mean temperature. 



As to geologic age, the primary requisite is that in some of the locali- 

 ties of plants in question their relative ages shall be determined by strati- 

 graphic evidence ; this being done in a few cases, it is not difficult to 

 assign to their approximate position intermediate or allied subfloras. 

 Plants treated in this way as evidence of geologic age have the advantage 

 of wide distribution over the surface of the land, of slow migration and 

 of long endurance in time. As in the case of animal fossils, we have to 

 allow for differences of station, for possible driftage and intermixture of 

 species belonging to higher and lower lands, and for chances of deposi- 

 tion and of preservation. We have also to consider that plants are more 

 permanent and less changeable than the animal inhabitants of the land, 

 and therefore better fitted, to mark the longer ages of geologic time ; but 

 this is more than compensated by the closeness of their relations with 

 the alternate elevations and depressions of our continents and the climatal 

 relations dependent on them. A single leaf of some plant of a temperate 

 genus found in arctic regions may thus bear explicit testimony to the 

 former geography of a whole continent, and the climatal phenomena de- 

 pendent on it ; and thus aid us in understanding and referring to its true 

 causes even the great Glacial period itself 



Glacial Period. 



I have recently been so venturesome as to add to the many publica- 

 tions on this vexed subject a republication of my numerous papers on 

 phenomena of the Glacial period in America ; and I am aware that many 

 of my friends in this Society will dissent very widely from the views 

 therein expressed. They will see, however, that I adhere very strictly 

 to the physical possibilities of ice, and to the doctrine of existing causes, 

 and that I have endeavored to take into account changes of geographic 

 forms, and of climate dependent on them, and of all the varieties of land 

 and water-borne ice anywhere to be seen in the colder portions of the 

 earth at present. It is, I am convinced, only by taking all of these into 

 account that we can succeed in explaining the complicated phenomena 

 of this remarkable age ; and we must be prepared also to allow for the 

 movements of elevation and depression which seem to have occurred in 

 that unsettled period, and of which many are fitted to produce a mini- 



