140 DUBLIN NATTTEAL HISTORY SOCIETT. 



by a corresponding theory of the mineralogical succession of igneous 

 rocks, which opens up a fertile field of speculation, hitherto unwrought. 

 The igneous rocks of the Palaeozoic period contain abundance of felspar, 

 whose principal constituent is potash ; the Mesozoic igneous rocks abound 

 in soda, replacing potash ; and in the tertiary period, soda itself gives 

 way to lime and magnesia. Viewed in the light of the Lamarckian phi- 

 losophy, here is a distinct indication that soda and lime are only allo- 

 tropic conditions of potash. "We may read the history of their formation 

 in the crust of the globe, if we will only open our eyes and see it written. 

 I may add, by the way, that this theory of the origin of lime is more 

 intelligible than that of many geologists, who would attribute the greater 

 accumulations of calcareous rocks in secondary and tertiary strata to the 

 creation of lime by organic force. 



If any chemist or mineralogist were to put forward such a geological 

 theory of the origin of soda and lime as the foregoing, he would be re- 

 garded as a lunatic by other chemists and mineralogists. 



How does it happen that a theory of the origin of species, which 

 rests on the same basis, is accepted by multitudes of naturalists, as if it 

 were a new gospel ? I believe it is because our naturalists, as a class, 

 are untrained in the use of the logical faculties which they may be cha- 

 ritably supposed to possess in common with other men. 'No progress in 

 natural science is possible as long as men will take their rude guesses 

 at truth for facts, and substitute the fancies of their imagination for the 

 sober rules of reasoning. 



It has been well observed by the greatest of living palasontologists, 

 " that past experience of the chance aims of human fancy, unchecked and 

 unguided. by observed facts, shows how widely they have ever glanced 

 away from the gold centre of truth !" 



