191 



Gill and the adjoining mine of Red Gill. In each case Anglesite 

 is associated with Cerussite, Pyromorphite, Mimetisite, Linarite, 

 and other ores of lead ; and it clearly represents one of the results 

 of decomposition of the Galena. Galena in the case of the ores 

 of lead, seems to indicate the high-water-mark (if one may so 

 phrase it), of the tide whose ebb is shewn by the decomposition- 

 products just mentioned. We can trace the course of the ebb 

 without much difficulty ; but our knowledge of the sequence of 

 events connected with the flow is very imperfect indeed. 



Mention has before been made of the limited distribution of 

 Anglesite as a decomposition-product of Galena as compared with 

 Cerussite. Mr. A. Dick informs me that the absence of Anglesite 

 in certain areas where Galena is undergoing decomposition is due 

 to the fact that the sulphate of lead arising from the oxidation of 

 the Galena is decomposed immediately it is formed if carbonate of 

 lime is present. The presence of Anglesite in the non-calcareous 

 rocks of the Lake District, and its absence in the area where the 

 mineral veins traverse limestone, is thus at once explained. 



Anglesite is found at very few other localities in the two counties 

 than the Caldbeck Fells ; and it is by no means of common 

 occurrence even there. 



Selenite, the crystalline form of the Hydrous Calcium Sulphate 

 Gypsum, or Ha Plaister, is a native product of some commercial 

 importance now-a-days. Occasionally, but very rarely, it occurs 

 as a decomposition-product in rocks, and in mineral veins ; but 

 the chief supplies of the mineral are obtained from beds occurring 

 in connection with the New Red — usually with some of the more- 

 argillaceous members of that series. Most of the Gypsum worked 

 in Cumberland and Westmorland is obtained from the shales 

 associated with, or on the horizon of, the ISIagnesian Limestone. Its 

 mode of occurrence seems to suggest, in some cases, that part of 

 it is not exactly contemporaneous with the particular beds where 

 it now occurs, but that (like the crystals of selenite occurring in 

 the London Clay, the Oxford Clay, and other similar deposits, 

 and like the chert and the flint in certain calcareous strata) it 

 represents, in a segregated form, mineral matter that was originally 



