182 On the origin of some 



neolite, and allied in composition to the zeolites, and these by 

 subsequent metamorphism have been changed into anhydrous 

 silicates. The production of harmotome, chabazite, and apo- 

 phyllite by the waters of a spring at Plombieres, at tempera- 

 tures not above 160° F. as observed by Daubree, lends probability 

 to such a view. 



But while we admit the possible direct formation of double 

 silicates in water at ordinary temperatures, there is not wanting 

 evidence that the reaction which we long since pointed out, (Proo 

 Eoyal Society of London, May 7, 1857) between silicious and ar- 

 gillaceous matters and earthy carbonates, in presence of alkaline 

 solutions intervenes in the metamorphism of sedimentary rocks 

 and in the production of many silicious minerals. The blue 

 Silurian limestones of the island of Montreal, when treated by acids 

 leave an insoluble residue, which contains about ten per cent, 

 of soluble silica, mixed with an argillaceous matter whose analy- 

 sis gave silica Y3*0, alumina 18*3, potash 5*5, and only traces of 

 lime and magnesia. In the vicinity of an intrusive dolerite, how- 

 ever, the limestone is changed in colour, and leaves by the action 

 of acids a greenish matter which consists of silica 40*2, alumina 

 9'3, peroxyd of iron 5*2, lime 36*6, magnesia 3.7. The free silica 

 and that of the intermingled aluminous silicate, has thus been satu- 

 rated with protoxyd bases, still however, retaining the alumina in 

 combination. A similar reaction with more aluminous matters, 

 would give rise to epidote, garnet, magnesian mica, scapolite or 

 feldspars like labradorite and anorthite, and it is not impossible 

 that in such reactions a portion of alumina may sometimes be set 

 free, and give rise to corundum, spinel, diaspore or volknerite. 



In the ordinary modes of decomposition of minerals containing 

 alumina, this base separates in the form of silicate, and the con- 

 ditions required for its elimination in a free state are but imper- 

 fectly understood. We have elsewhere pointed out the decom- 

 position by alkaline and earthy carbonates, of solutions of sul- 

 phate of alumina or native alum, as one source of free alumina, 

 and insisted upon the existence of pigotite, a native compound of 

 alumina with an organic acid, as an evidence that this base is 

 sometimes like oxyd of iron, (and oxyd of manganese,) taken 

 into solution by water aided by organic matters. A hydrate of 

 alumina gibbsite, is found associated with limonite, and the 

 aluminous minerals from the south of France described by Ber- 

 thier and Deville, show that free alumina is much more common 



