116 ROCKS OF* THE TR AFP-FOK M ATION. 



turally led to the idea of an increafing finencfs in the folution, 

 becaufe we obferve mofl diflinclly that the outer coat is com- 

 pofed of the coarfcft precipitate, and that as it approaches the 

 middle, it is always finer and finer ; fo that at laft, out of the 

 rineit folution, amethyft (hoots, and, when there is room enough, 

 whence the pre- chryftallizes. That the folution out of which the precipitation 



aXs^Vph™" took P Iace was trul y chemical > is demonftrated not only by the 

 character of the fubitances, but alio by the difpofition of the 

 coats ; for we can obferve that they have followed the attrac- 

 tion of the walls of the cell, and are fo accurately depofited 

 upon it, that an inequality, or elevation upon the outer wall, 

 continues to be obferved through all the fuperincumbent coats 

 to the center of the agate. That the folution had come from 

 without, and by infiltration penetrated into the cells, is molt 

 diftincily to be icon in fpecimens of agate when they are pro- 



The opening of perly cut. Ii' the cutting has been judicioufly done, we can al- 



in filtration ob- W ays obferve the opening by which the folution has entered. 

 This elegant explanation is an outline of that delivered by the 

 illuftrious Werner in his geognoitieal lectures, and is a demon- 

 itration of the aqueous origin of agate. 



Flinty foffils not That flinty foffils, as flated by Mr. Playfair, are impervious 



impervious to ^ Q wa £ er ^ j ias never been proved ; on the contrary, it is well 

 known to collectors of fofiils, that if the cellular rock chryftal, 

 which contains water, is kept in a cabinet for fome years, the 

 water gradually difappears. 



Other obf. Sections 249, 250, 1, 2, are occupied with an unfuccefsful 



attempt to reconcile the appearances on the hill of Scheibenberg 

 with the Huttonian theory. Without innfting upon the Wer- 

 nerian doctrine of formations, which gives the death-blow to 

 all partial explanations like thofe propofed by the Huttonian 

 theory, I ihall merely mention a few fads respecting the indivi- 

 dual relation of thefe rocks, to flievv the infufheiency of the exr 

 planation attempted by ProfeiTor Playfair. 



Situations of 1. Sandftone is fometimes obferved covering wacken. 



wacken admitted 2 Gravel cover ed by flaty clay and bafalt, has been ob- 



by Prof. P. rjl . , J J J 



lerved lying on wacken. 



3. Veins of wacken fometimes traverfe gneifs, porphyries, 



fandflone, &c. 



adduced to prove Now, if wacken is clay hardened by the fuper-pofUion o£ 



hardeneTb "fire. melted balalt ' VVG mufl fu PP ofu the lume effcCi to have ^^ 



4 produced 



