SOME UNSOLVED PROBLEMS IN GEOLOGY. 67 



dismembered. The writer has himself done something toward this,* 

 but Professor Nathorst has done still more ; f and now some intelligi- 

 ble explanation can be given of many of these forms. Quite recently, 

 however, the Count de Saporta, in an elaborate illustrated memoir,J 

 has come to the defense of the fucoids, more especially against the 

 destructive experiments of Nathorst, and would carry back into the 

 vegetable kingdom many things which would seem to be mere trails 

 of animals. "While writing this address, I have received from Pro- 

 fessor Crie, of Rennes, a paper in which he not only supports the algal 

 nature of rusichnites, arthrichnites, and many other supposed fucoids, 

 but claims for the vegetable kingdom even receptaculites and archaeo- 

 cyathus. It is not to be denied that some of the facts which he cites, 

 respecting the structure of the siphonise and of certain modern incrust- 

 ing algse, are very suggestive, though I can not agree with his conclu- 

 sions. My own experience has convinced me that, while non-botanical 

 geologists are prone to mistake all kinds of markings for plants, even 

 good botanists, when not familiar with the chemical and mechanical 

 conditions of fossilization, and with the present phenomena of tidal 

 shores, are quite as easily misled, though they are very prone, on the 

 other hand, to regard land-plants of some complexity, when badly pre- 

 served, as mere algae. In these circumstances it is very difficult to 

 secure any consensus, and the truth is only to be found by careful 

 observation of competent men. One trouble is, that these usually 

 obscure markings have been despised by the greater number of paleon- 

 tologists, and probably would not now be so much in controversy were 

 it not for the use made of them in illustrating supposed phylogenies 

 of plants. 



It would be wrong to close this address without some reference to 

 that which is the veritable pons asinoTum of the science, the great 

 and much-debated glacial period. I trust that you will not suppose 

 that, in the end of an hour's address, I am about to discuss this vexed 

 question. Time would fail me even to name the hosts of recent au- 

 thors who have contended in this arena. I can hope only to point out 

 a few landmarks which may aid the geological adventurer in travers- 

 ing the slippery and treacherous surface of the hypothetical ice-sheet 

 of pleistocene times, and in avoiding the yawning crevasses by which 

 it is traversed. 



No conclusions of geology seem more certain than that great 

 changes of climate have occurred in the course of geological time ; 

 and the evidence of this in that comparatively modern period which 

 immediately preceded the human age is so striking that it has come 

 to be known as pre-eminently the ice age, while, in the preceding ter- 



* " Footprints and Impressions on Carboniferous Rocks," " American Journal of Sci- 

 ence," 1873. 



f Royal Swedish Academy, Stockholm, 1881. 

 :f " A propos des Algues Fossiles," Paris, 1883. 



