XIII.] ORIGIN OF CRYSTALLINE ROCKS. 313 



APPENDIX. 



REPLY TO MR. DANA'S CRITICISMS. 



IN the American Journal of Science for February, 1872, Professor 

 Dana has criticised certain points in my address, On the Geognosy 

 of the Appalachians and the Origin of Crystalline Rocks, given in 

 August, 1871, at Indianapolis, before the American Association for 

 the Advancement of Science. I am charged by him with rejecting, 

 for many mineral silicates, the view that they are pseudomorphs ; 

 that is to say, crystals chemically altered without loss of external 

 form. I have denied that crystals of serpentine having the shape 

 of chrysolite, pyroxene, dolomite, etc., and crystals of pinite having 

 the shapes of nepheline or scapolite, are results of a chemical 

 change of these species, nothwithstanding this view is now held 

 by most mineralogists, on the grounds of similarities of geometrical 

 form and the existence of what are regarded as intermediate stages in 

 the process of transmutation ; and I have maintained another and 

 a very different view, which, in my opinion, is more rational. Until 

 we can watch the transmutation of one of these species into another, 

 the argument from supposed intermediate forms is worth no more in 

 the mineral than in the organic world ; the reasoning of the trans- 

 mutatioirists, in the one case and the other, resting upon somewhat 

 similar considerations. In either case we may say, with Professor 

 Warrington Smyth, that in these intermediate forms " lie the ma- 

 terials for a history " ; while we venture, with him, to express a 

 doubt whether, from a series of specimens supposed to show a 

 transition from chrysolite to serpentine, or from hornblende to 

 chlorite, " we are obliged to conclude that there has been, histori- 

 cally speaking, an 'actual transition from the one to the other." (See 

 his anniversary address, as President of the Geological Society of 

 London, in 1867.) 



Professor Dana says that Scheerer is the only one who shares my 

 peculiar views on this question. I have, however, asserted in my 

 address that Delesse has maintained the views of Scheerer and 

 myself, as opposed to the popular doctrine of epigenesis, and shall 

 endeavor to make good my assertion. In his essay on Pseudo- 

 morphs, published in 1859 (Ann. des Mines (5), XVI. 317-392), 

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