Clifton College Scientific Society. 49 



occasionally on the floor of a cavity are seen stalagmites rising to 

 meet the stalactites above, and these stalagmites are short and 

 broad, just as they are in limestone caverns. Crystals of quartz 

 occur on these floors, generally lying flat, and sometimes they 

 have groups of crystals of goethite on their upper flat surface, a 

 mode of occurrence which I have not seen in the quartz crystals 

 on the stalactites above. 



The minerals associated with the iron ore are quartz and 

 sulphate of barium. 



The quartz occurs disseminated through the hgematite, filling 

 up small cavities of quite irregular shape, and also in regular 

 crystals on the surface of the stalactites in the cavities. 



I have not yet seen any amethystine or purple quartz in this 

 deposit, nor any large crystals, though the finest amethystine quartz 

 in England has been found in Providence iron mine, at Ashton. 



There are, however, here some very interesting crystals, though 

 of small size. These are clear quartz crystals, in the interior of 

 which are seen perfect models of the crystals of an opaque pale 

 yellow or orange colour, the formation of which may be explained 

 as follows : — 



At some period when the quartz crystals were smaller, an excess 

 of iron was introduced by the water flowing through the cavity, 

 and this deposited a thin coating of hydrated oxide on the crystals 

 already formed ; this coating, however, was not sufficiently thick 

 to affect the growth of the crystal, and as the water cleared, 

 successive layers of colourless quartz were added till the crystals 

 grew to their present size. 



As a rule, the silica has been deposited after the iron ; for the 

 crystals generally adapt themselves to the stalactites, being fixed 

 on to them, and in some cases a crystal in growing has met and 

 enclosed the end of a stalactite already formed. The crystal some- 

 times is bifurcated, and terminates in a double instead of a single 

 pyramid, leaving the stalactite between the two pyramids. This 

 rule is not, however, invariable, for cases occur in which a stalac- 

 tite growing downwards has met a quartz crystal and has been 

 thrust more or less out of its course. 



The quartz crystals attached to stalactites are generally hexagonal 

 prisms terminated at each end by a pyramid ; often, however, they 

 are attached to the stalactite by one end of the prism. Cavities 

 occur in the sandstone also lined with quartz crystals very short 

 and crowded together, owing to the abundance of silicious matter 

 in solution. 



Some of the crystals are so transparent that they are seen with 

 difficulty, but the majority reflect white light from some part of 

 the interior. 



