Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/187

 Rh rock is decomposed, it may be concluded that ore will very soon be found. The arsenic acid, according to Werner, produces a similar alteration upon rocks, and the carbonic acid also appears sometimes to contribute to this decomposition. It is in porphyritic rocks, sienite, gneiss, mica-slate, and clay-slate, that this alteration principally occurs, but it is still more distinctly seen, where felspar is found, as has been said above in noticing the veins of granite, which have been found at a great depth in some of the mines of Cornwall.

F. Of the different Matrices accompanying the mineral Deposits in Cornwall and Devonshire.

By saalbande is understood a slip or band interposed between the vein and the rock which forms the body of the mountain. The