■with Chemistry and the Natural Sciences. 3S3 



ment of the decomposition, so that an approximate vaUie may 

 be procured concerning tlie epoch of this last phenomenon. 

 An observation of this sort was made at Limoges in the year 

 1833. The cathedral of that town was built about four cen- 

 turies ago of granite, which must have been procured from the 

 quarries which were nearest to the town. In the interior of the edi- 

 fice, the granite is scarcely if at all altered ; whilst on the outside 

 those places exposed to the rains and the winds, the decomposi- 

 tion and disintegration is, in some portions, pretty deep ; in 

 other parts again it is much less ; so that the mean alteration 

 may be valued at 8 millimetres. But the portion of the granite 

 mass decomposed in the quarry is l'",62. If, then, we sup- 

 pose that the progress of the alteration in the mass of granite 

 took place in the ratio of the time, the alteration must needs 

 have commenced about eighty-two thousand years ago. It is 

 true, that we are yet in a great degree ignorant of the real pro- 

 gress of the decomposition of granite in the mass ; but it must 

 assuredly have been more rapid in earlier than in later times, 

 when the upper portions must have to a certain extent preserved 

 those which were underneath. In this case the law must have 

 a decreasing progression, and would yield a still higher number 

 than that just quoted. 



We have verified the supposition, that the alteration of the 

 granite of the cathedral is of the same kind as that of the gra- 

 nite in masses ; it results from the decomposition of the super- 

 ficial portions of the felspar, which, by losing its silicate of 

 potash, leads to the disintegration of the constituent parts of 

 the granite. We need scarcely remark, that our chief object in 

 supplying these results, is for the purpose of exciting geologists 

 to pursue this kind of investigation, which may one day lead to 

 the discovery of data which will prove valuable in elucidating 

 the history of the earth. 



The study of these very gradual operations has a bearing, the 

 extent of which is far from being known, not more in a scien- 

 tific, than in an economic point of view. The application which 

 is about to be made of it in the extraction of some metals from 

 their respective ores, particularly of silver, may here be no- 

 ticed in illustration. The process most generally and longest 

 employed in the management of minerals containing silver, con- 



VOI,. XXIII. NO. XI.VI. OCTOBER 1837. Y 



