186 Scientific Intelligence—Mineralogy. 
11. On the Decomposition of Trap-Rocks. By M. Ebelmen. 
—(1.) A trap from near St Austle (Cornwall). This trap consists 
essentially of Labradorite and Pyroxene. 
Trap fresh or Altered Trap more 
unchanged. trap. altered. 
Alumin,  . . 100 100 100 
Silica, - ; : 325 212 201 
Lime, : ; : 36 5 6 
Magnesia, : , 17 14 12 
Oxide of iron, . , 106 107 79 
Oxide of manganese, 3 2 
Potassa, . b aby): Ws 
Beit walked simdos btiek, me ty 
Water, . : : ial 43 38 
631 497 449 
Hence the trap, by decomposition, has lost more than a third of its 
silica, & of the lime, and half of the alkalies; this last shews that 
the feldspar was the last to change, and had not been wholly decom- 
osed. 
(2.) A Basalt from the Rhine, consisting of Labradorite, about 54 
per cent., Pyroxene 24, Chrysolite 10, with titanic iron 10, and 
water 2 per cent., afforded him— 
Unchanged Basalt 
Basalt. altered. 
A. B. 
Alumina, ; 5 P F 100 100 
Silica, Fs s ‘ : : 285 228 
Lime, F 5 : é . 63 43 
Magnesia, ‘ ; : . 39 29 
Oxide of iron and manganese, . 80 78 
Titanic acid, . ‘ - : 6 6 
Potassa, . : : ; : 7-4 2°6 
Soda, : 5 ; é é 22:2 74 
Water, . ; ‘ : * 15:0 35°0 
615°6 529°0 
Here two-thirds of the alkalies have disappeared, shewing that the 
decomposition of the feldspar was far advanced, The result of the 
changes, in both cases, is to produce, as the residue, an hydrated sili- 
cate of alumina, or a clay. The removal of the silica is shewn by 
M. Ebelmen to be independent of the alkalies present. The decom- 
position is attributed by him to carbonic acid and oxygen present in 
waters, to organic matters living, or in course of decomposition, and 
the phenomena of nitrification —(Annales des Mines, 12, 627.— 
Silliman’s Journal, vol. viii., No. 24, p. 421.) 
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