148 Professor Keilhau on Contact Products, 



resemblance alone is taken into consideration, and not the 

 arbitrary requirements of particular systems. 



An opinion which has been really expressed can also be 

 brought forward respecting the phenomenon near Plauen. 

 The observer who described the appearance says, that the 

 hornstone-like and fine granular nature of the limestone mass, 

 leads to the conclusion, " that a complete chemical penetra- 

 tion of carbonate of lime mixed with siliceous jelly into the 

 fissures of the syenite, has taken place." It is, thus, not here 

 assumed that the peculiar constitution of the Fldner forma- 

 tion, where it is in the vicinity of the syenite, belongs to 

 the usual contact-actions ; and why % Most assuredly because 

 the syenite cannot be asserted to be newer than the Planer, 

 or, in other words, because it is impossible in this case to assert 

 that the syenite operated by heat. On this account, the above 

 unnatural hypothesis was had recourse to, and thus it is proba- 

 bly assumed that the proper homage due to chemistry has 

 been rendered. If I am not mistaken, we have here an ex- 

 ample which, though otherwise of little moment, is yet very 

 instructive, from its exhibiting, in a striking manner, to what 

 the method at present employed often leads ; viz. to misap- 

 prehension and suppression of important facts, and to the 

 formation of artificial ideas, rather resembling a profitless play 

 of fancy, than the anxious endeavour to discover the truth. 



The contact phenomenon in Auvergne also receives no 

 particular attention from the volcanists, as it is here impossible 

 to cite the granite in a hot state as its cause. 



I shall here give a few of my own observations, chiefly for 

 the purpose of shewing, that a complete study of the silicifica- 

 tions occurring in certain rocks, requires the consideration of 

 a number of, as it would seem, very complicated relations, 

 which, notwithstanding their importance, enquirers may be 

 very easily induced to overlook, under the guidance of the 

 existing school : 1. Contact indurations often do not exist at all 

 in the vicinity of masses which are not less regarded as pyro- 

 genic by the prevailing school, than others near which these 

 changes are met with in a high degree of development. Thus, 

 such indurations are not observable in the Christiania district 

 near those masses of porphyry and greenstone which have more 



