146 Professor Keilhau on Contact Products. 



gard to the Auvergne granite, it certainly cannot be supposed 

 by any one to belong to the tertiary or to a still newer epoch ; 

 and if it ever was hot, this must have been very long before 

 the deposition of the strata which have been modified by it. 

 It will hence be concluded, that such changes as those spoken 

 of can go on at the ordinary temperature. It is next to be 

 considered, whether more general information is to be de- 

 rived from the same facts. Unfortunately, the result will so 

 far not be a brilliant one ; but, as I have repeatedly said, the 

 reasonable investigator does not expect this. Here, again, 

 we have effects of actions which are, for the most part, only 

 exhibited in one or other of such effects produced long ago, 

 and, in trying to account for which, while we are groping but 

 too much in the dark, we are again reduced to call upon 

 electro-chemical currents, molecular displacement, cementa- 

 tion, &c. It may possibly be considered as essential, that the 

 masses by which the silicifications have apparently been effect- 

 ed, are themselves very rich in silica. It is to be remarked, 

 that the inquirer, by having obtained these moderate results, 

 has still reached a point, whence he cannot be so easily led 

 into error by the discussion of other facts connected with the 

 subject. For example, should any one, in order to support the 

 hypothesis of the volcanists, refer to the thousands of places 

 where silicification has taken place in the vicinity of rocks 

 which, according to the prevalent opinion, are pyrogenic, the 

 answer would at once be, that even though this view were cor- 

 rect, still there is no proof that the heat had caused the change ; 

 nay, if due weight be given to facts like those adduced, it must, 

 on the contrary, be concluded, that this change near the really 

 pyrogenic rocks has likewise happened in their cold condition. 

 But let us now return to our examples, in order to see how 

 the supporters of the prevailing school would treat, or have 

 really treated of them. The first of them will probably 

 be dictatorially rejected as inapplicable ; or, at all events, it 

 will be exclaimed, this is a mere bagatelle, an isolated fact, 

 which is not to be taken into consideration ! As it is always 

 the practice to endeavour to give the method followed an air 

 of strictness, the principle is announced, that conclusions are 

 only to be formed from the totality of the observed facts ; but 



