BY H. F. WALLMANN. 



33 



It is not the mere force of its pressure that is able to broak up 

 the obdurate resistance of compact limestone or quartz, or 

 of even the glassy mass of obsidian or pitchstone, or of the beJs 

 of unctous clay ; it is its constantly renewed impact, which, 

 together with other causes, gradually increases the means of 

 infiltration by the fissures, and at the same time intensifies the 

 chemical disturbances which it commenced. 



To this we trace the origin of all zeolites, and of most 

 decomposition products, which subsequently appearing as specific 

 minerals, are again liable to be similarly affected by that circulation 

 of water to which their existence was due. 



What wonder then that we find in the cavities of basalt the 

 water-born zeolites — in nuggets of melaphyre amethyst, or in 

 limestone caves the pendent stalactites. 



But not to go out of our way to explain the various theories 

 respecting the action of water on the rocks, we will turn at once 

 to the phenomena of decomposition and recomposition which are 

 illustrated by the minerals under notice. 



Take first of these the cosmopolitan mineral. Cuprite or red 

 oxide of copper, Cu 2 O, with tesseral crystals appearing as cubes, 

 octahedrons, dodecahedrons, or in combinotions and twins after 

 octahedrons. The colour is cherry, or different shades of red, the 

 streak brownish red. 



Next, a green mineral, Atacamite, a hydrous oxy-chloride of 

 copper, H Cu 2 3 CI. The crystals are rhombic in short or 

 long prisms. They occur in groups, and massive lamellar. The 

 colour is dififerent shades of green to blackish green. Translucent 

 to semitransparent, with vitreous lustre and apple-green streak. 

 The origin of this decomposition product of copper is briefly due 

 to the action of rain or spring water holding chlorine or chlorides 

 in solution. The tesseral Cu 2 passes by substitution into the 

 rhombic H 3 Cu 2 O 3 CI, with a loss of hardness and specific 

 gravity. Wherever there are copper ores and chloriferous water 

 this reaction is to be expected, either complete or incipient, but 

 near the surface or in the fissures of the ores. 



A third product of alteration is Malachite, a hydrous carbonate 

 of copper, Cu 2 CO 4 + H2O, of a green colour and with a green 

 streak. The crystals are monocline, generally microcrystalline, 

 needle or hair-like, in thin plates, racemose or kidney shaped, and 

 the chemical composition is 72 CuO 20 CO 2 + 8 H 2 O. Where- 

 ever copper occurs Malachite a[)i)ears under the influence of the 

 carbonates contained in air or water. 



