Mr. R. W. Fox on Pseudomorphous Crystals of Quartz. 7 



crystals to have been again repeated : then came a coat of 

 silex over the fluor, or judging from the lines, many coats of 

 it, forming a thick crust, having a surface of small quartz 

 crystals. Some specimens were found at the same time with 

 one or more layers of quartz between two or more portions 

 of fluor, which tend to confirm these views. 



I think it may be inferred, from the well-defined and smooth 

 impressions which the octahedrons of fluor have left in the 

 quartz, and the general parallelism of the sides and angles of 

 the outer cavities to those of the smaller pseudomorphous 

 crystals inclosed in them*, that the inner and outer crystals 

 of fluor were perfect and uninjured until after the whole se- 

 ries of them were coated with quartz. At some subsequent 

 period then it would appear that other changes occurred in 

 the vein, and that the solution or destruction of the fluor com- 

 menced. Some of the cavities which were found to contain 

 water only, as well as those which contained water together 

 with disintegrated fluor, have the appearance of having been 

 so hermetically sealed, that it is difficult to understand how 

 the liquid solvent could have obtained access to the fluor and 

 abstracted it from its case. It cannot be supposed that the 

 pressure of the column of water above it, although equal to 

 more than half a ton on some of the larger crystals, could 

 alone have produced the effects ; for not only must the solvent 

 have been continually admitted through the crusts of the 

 quartz, but the salts resulting from the solution of the fluor 

 must, at the same time, have passed through thSm in the op- 

 posite direction, — a sort of endosmose and exosmose must have 

 existed, as I conceive, to produce the phaenomena ; whilst in 

 other instances, the thick envelopes of quartz were impervious 

 and protected the fluor from injury. The salts resulting from 

 the solution of the fluor must have been soluble, although this 

 condition seems to present some difficulties under the circum- 

 stances of the case ; and doubtless the destruction of the fluor 

 was very slowly effected in many instances, and in others it 

 was begun, but never completed. The differences in the saline 

 contents of the water obtained from some of the crystals is an- 

 other circumstance of some interest, indicating the existence 

 of different conditions in the vein when the water was last ad- 

 mitted into the respective crystals. 



The phaenomena exhibited by these minerals cannot, I con- 



* How are such coincidences to be accounted for? Are we to assume 

 that jwlarising forces have determined the arrangement ? In many instances 

 the layers of quartz which were interposed between the crystals are very 

 thin, imperfect, and pervious to water; but in others they are not so, and 

 some of the inner crystals now contain water. 



