192 



also near Keswick, and occasionally at Alston as well. Some 

 beautiful glassy crystals of Aragonite, occurring on Dolomite, have 

 been obtained from Nent Force, at the last-named locality. 



In regard to the genesis of Aragonite, where it occurs in intimate 

 association with the Hexagonal form of the same chemical com- 

 pound, no very satisfactory explanation has yet been given. 

 Differences in the temperature, or in the nature, of the depositing 

 solution, may have determined whether the Calcic Carbonate in 

 particular cases should crystallize in the one system or in the other ; 

 but these explanations will hardly apply in the cases where both 

 crystalline forms are found associated in such a manner as to 

 suggest that they have crystallized simultaneously, and under 

 precisely the same conditions. The subject seems as yet to be 

 but imperfectly understood, as is that of the related subject of the 

 distribution of particular crystalline forms in regard to locality. 

 Further reference will be made to this under Calcite. 



WiTHERiTE, Barium Carbonate, occurs in a few localities ; 

 chiefly in the lead-mining areas of the Lower Carboniferous rocks. 

 In most cases it occurs in connection with faults, where they 

 traverse, or are connected with, beds of limestone; and it is 

 usually found in masses in the rider, or fault-breccia, of mineral 

 veins. In the majority of instances it appears to be associated 

 with Barytes and the other minerals above mentioned as occurring 

 along with Aragonite. This mineral crystaUises in the Rhombic 

 System ; but quite commonly the fundamental form is modified in 

 such a manner as to closely approach some of the forms belonging 

 to the Hexagonal System. Thus, some of the crystals of Witherite 

 occurring at Alston, as well as at Dufton and the parts adjoining, 

 occur in the form of pseudo-hexagonal pyramids, very much Hke 

 one of the commonest crystalline forms of quartz. Associated with 

 crystals of this kind are others consisting of groups of rod-like 

 aggregations, each individual crystal of the aggregation consisting 

 of a combination of the two terminal planes and the double six- 

 faced rhombic pyramid, so that each rod is made up of several 

 crystals joined end to end like a string of beads. Some of the 

 Witherite occurring at Dufton is in the form of ball-shaped masses, 



