Professor Keilhau on Contact Products. 145 



limestone reposes on granite and gneiss. As the fundamen- 

 tal rock, during the deposition of the new masses, was full of 

 open fissures and rents, the limestone is also found in these. 

 Thus the limestone is very frequently in contact with the si- 

 liceous rocks. It is there to be seen firmly cemented to the 

 subjacent mass, and possessing great hardness ; in other words, 

 it has become siliceous. 



3. In the Plauensche Grund, near Dresden, the Planer for- 

 mation reposes on a syenite, which is held to be undoubtedly 

 of older origin. At certain points, the former has filled up 

 fissures in the syenite, and in these fissures, at least, the lime- 

 stone has been found converted into a fine granular mass re- 

 sembling hornstone. 



4. In Auvergne, it has been observed that tertiary strata, 

 which lie on granite (a granite which in many places passes 

 into gneiss), possess a greater degree of hardness near that 

 rock, and that the new masses and the granite are so much 

 intermingled at their common boundary, that it is difficult to 

 distinguish them. (Bull, de la Soc. Geol., t. xiii. p. 220.) 



These facts will be treated in a different way by diff"erent 

 geologists. He who in his investigations sincerely wishes to 

 obtain a correct result, even though it should be in opposition 

 to this or that system, will first of all, without having the 

 smallest doubt on the subject, recognise all these facts as be- 

 longing with perfect justice to the category in question. We 

 may assume that, in the next place, in the problem regarding 

 the cause of these contact phenomena, he will test the hypo- 

 thesis of the volcanists. If he be but convinced of the accuracy 

 of the data adduced, he must soon become aware how little 

 applicable in these cases is that hypothesis. As to the first 

 example, the masses near which the silicification has taken 

 place, are of such a nature that no one supposes that they 

 have been in a melted state, or have had a high temperature ; 

 and in both the two next instances, the altered deposit is in 

 contact with rocks, which, if they were ever in a hot condition, 

 undoubtedly were not so at the time when the new masses were 

 in juxtaposition with them. When the superimposed de. 

 posits were formed, these rocks presented an ordinary, wea- 

 thered, fissured, and undoubtedly very old surface. With re- 



VOL. XXXVII. NO. LXXIII. JULY 1844. K 



