174 Mr. A. Gages on a Method of Observation 



like it, was composed of a series of parallel lamiuse : the interior 

 layers possessed a certain amount of permeability, which, upon 

 examination with a lens, showed that silica had passed into the 

 crystalline state ; nevertheless some traces of amor])hous silica 

 could still be detected by the test of caustic potash. The density 

 of this pseudomorph is nearly the same as that of ordinary 

 quartz-rock. It would appear, therefore, that the alumino-mag- 

 nesian base of the original rock having disappeared, a perme- 

 able siliceous skeleton remained, which was subsequently infil- 

 trated by the silica of the alkaline silicates derived from the de- 

 composition of the surrounding trap-rocks. 



To the same class of phaenomena we may probably also refer 

 the petrifaction of the fossil wood occurring in the vicinity of 

 Lough Neagh, and which, according to Bischof, contains 71 per 

 cent, of silica ; and the slight alkaline reaction which the same 

 observer has attributed to the waters of that lake, may in such 

 case be attributed to the decomposition of the alkaline silicates. 



The mineral substances called mountain leather and mountain 

 cork, which are chiefly derived from the decomposition of horn- 

 blendic and augitic rocks, as in the county of Londonderry, 

 exhibiting as it were in themselves a kind of natural process 

 similar to that here described, leave, when treated with acid, a 

 white spongy skeleton of excessive lightness, which swims in 

 water and bears the greatest analogy to some varieties of nectique 

 quartz. This residue of silica absorbs about four times its 

 weight of water, and rapidly dissolves in a weak solution of 

 caustic potash, even after the skeleton has been heated to redness. 

 The specimens of mountain leather, cork, &c., from the district 

 above named, as well as many minerals of a like character, are 

 evidently the result of a more or less advanced state of alteration 

 of hornblendic and augitic rocks. We may recognize two stagesof 

 this decomposition : in the first, we have sometimes almost a 

 mere spongy aluminous silicate, of variable composition, often 

 more or less impregnated with carbonates of lime and magnesia, 

 or with carbonate of lime alone ; in the second stage the whole 

 of the siliceous compounds forming the sponge disappear, and 

 are replaced by carbonates of lime and magnesia, or by both — 

 the character of the metamorphosis being often only recogniz- 

 able by thin coatings, often mere films, of mountain-leather 

 substances, covering one or both sides of the replacing carbon- 

 ates : these films are always recognizable by a practised eye. 



I may here remark that the silica retained in the pi-eceding 

 substances is always in the soluble amorphous condition, which 

 appears to be conclusive as to their origin. 



Serpentine, cut into thin pieces of various shapes and treated 

 by acids and other solvents, exhibits in a great number of in- 



