ITS MATERIALS— THE LONDON CLA V. 27 



filled up by calc-spar deposited by its crystallization from 

 solution in water which has reached them by percolation from 

 the exterior. Septaria, therefore, on being cut through, present 

 a number of divisions bounded by sej>^a (hence the name) or 

 partitions of calc-spar, and the hard substance forming the mass 

 between taking a dull polish, some are available for small 

 ornamental table-tops. 



Beautiful crystals of Selenite or crystallized sulphate of lime 

 are also very abundant in the more exposed portions of the 

 clay, where this substance is separated from the clay, or rather 

 produced, by a natural chemical process. The crystals are often 

 almost transparent, and sometimes perfectly so, frequently double, 

 and quite regular in form, and are usually in rhomboidal prisms 

 with bevelled sides. Selenite is the crystalline form of gypsum 

 and alabaster, and is much softer than calc-spar, which is the 

 corresponding crystalline form of carbonate of lime. 



Iron Pyrites, a compound of sulphur and iron, although 

 not an abundant mineral in the clay of Hampstead, is so in 

 the London Clay of other areas, especially the Isle of Sheppey, 

 where, from the rapid destruction of the cliffs by the sea, it is 

 collected in sufficiently large quantities from the shore to supply 

 a manufactory of copperas. It frequently encrusts fossils, but 

 often quickly decomposes on exposure, and so destroys the 

 specimens. This mineral is very abundant in Nature, and 

 sometimes occurs in beautiful crystals, and so gold-like that they 

 are often eagerly secured under the belief that they contain 

 the precious metal. Iron Pyrites is, however, of little com- 

 mercial value, and is not used as an ore of iron. 



During the progress of the Archway excavation at Highgate, 

 a curious soft resinous mineral was found in the London Clay, 

 and called " Highgate Resin," and from its resemblance to the 

 gum copal, '' Copaline." 



