1835.] BecquereVs Traitt Experimental, 8fc. 73 



potash disappearing and silicate of alumina remaining. Felspar is 

 probably one of those bodies whose particles are placed in such inti- 

 mate union that acids have no effect upon it until it be exposed to 

 electro-chemical agency. Fournet has observed three preliminary 

 stages in the decomposition of granite. 1. A superior zone of a red 

 or yellow colour, indicating the peroxidation of iron. 2. A middle 

 zone of a deep green colour. 3. An inferior zone, presenting all 

 the characters of a perfect granite, but falling to pieces when touched. 

 He accounts for the successive decomposition from the surface, inter- 

 nally to dimorphism, which has changed their crystalline texture 

 like arragonites and laumonites. Gustav. Rose has produced 

 pyroxene and amphibole as instances of this dimorphism, of which 

 some result from rapid, others from slow cooling. The theory of the 

 felspar decomposition Fournet sums up shortly. The iron is per- 

 oxidized, carbonic acid is absorbed and takes the place of the silica, 

 which, being set at liberty in a gelatinous state, dissolves in water, 

 or alkaline carbonates, and gives origin to crystals of hyaline quartz, 

 iorites, agates, opal, calcedony, and silicates, as chabasite, mesotype. 



This theory, however, rests upon two suppositions which have not 

 yet been demonstrated. 1. That igneous rocks do not acquire a state 

 of permanent equilibrium, and that they exhibit in the course of time 

 an effect of dimorphism, and 2. that carbonic acid is absorbed by 

 these rocks. The latter appears to be strongly exhibited in Auvergne, 

 where numerous mineral springs, which escape from granite fissures, 

 act upon the rocks, and form small irregular basons which they fill 

 with hydrous peroxide of iron. 



Sparry iron ore. — Granite before it decomposes disintegrates, 

 but the iron ore retains its form, and yet changes its chemical nature. 

 Becquerel has examined the process of the decomposition of this 

 mineral in Isere, and he has found it entire when preserved from the 

 contact of air and water. In Dauphine it is decomposed in such a 

 manner as to give out heat and light, which burst into flame and 

 continue to burn. The inhabitants regard the presence of these 

 flames as a decided proof of the existence of rich mines of this 

 mineral. The mineral contains carbonate of manganese and mag- 

 nesia. The iron and manganese change into hydrates, lose their 

 carbonic acid which combines with the magnesia, and renders it 

 soluble in water. Water is decomposed to afford oxygen to the 

 hydrate, and the hydrogen inflames after overcoming an immense 

 pressure. 



According to Chapert, when some of the minerals accompanying 

 this iron ore are roasted, and left to spontaneous action, after some 

 days, sulphate of magnesia and iron, and carbonate of copper appear, 

 facts of great importance in electro-chemistry. Four kinds of pyrites 

 accompany this ore, which give origin, to 1 . Neutral sulphate of iron. 

 2. Earthy sulphate, a yellow substance, resinous or earthy. 3. Ochre 

 proceeding from the action of air upon the neutral sulphate ; besides, 

 sulphate of iron and alumina, manganese, lime, zinc, &c. 



Lavas. — Granite decomposes readily in contact with bay-salt, as 

 is evinced in Scotland and Clermont. The facility of the decompo- 

 sition of lavas varies with their composition ; thus the pyroxenic rocks 

 of Auvergne decay more rapidly than the Labradore masses of 



