. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. CONTACT METAMORPHISM 137. a quartzite of this nature is shown in Fig. 13. In this case the original rounded granules are readily recognized from the fact that not merely did they contain small cavities and needle-like enclosures, but exteriorly they were cov- ered with a thin pellicle of iron oxide, while the secondary deposit, which now fills all the inter- spaces, is free from en- closures of all kinds and quite pellucid. In many quartzites a shearing force has acted a prominent part, where- by the granules have be- come el
. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. CONTACT METAMORPHISM 137. a quartzite of this nature is shown in Fig. 13. In this case the original rounded granules are readily recognized from the fact that not merely did they contain small cavities and needle-like enclosures, but exteriorly they were cov- ered with a thin pellicle of iron oxide, while the secondary deposit, which now fills all the inter- spaces, is free from en- closures of all kinds and quite pellucid. In many quartzites a shearing force has acted a prominent part, where- by the granules have be- come elongated and more or less pulverized along ^'?- IS-Mierosfcructure of quartzite, ^ow- , . , ^ mg secondary deposit of silica about tne tneir margins by tne original quartz grains, friction of rubbing one over the other. In such cases mica and other secondary min- erals are often developed, and the rock passes over into a mica schist. Still another form of change, or metamorphism, is that known by the name of metasomaiosis, a process of indefinite substitution and replacement. Through the chemical action of percolating solutions certain constituents of a rock may be leached out and replaced by others in indefinite proportions- It is by such processes that have originated a large share of the serpentinous rocks, dolomites, etc. The mineral olivine, an anhydrous ferruginous silicate of magnesia, passes over into serpentine by a simple process of hydration, and a more or less complete change of its combined iron from the ferrous to the ferric state. Provided there be no loss in silica, this change in the olivine, according to T. Sterry Hunt, must be accompanied by an increase of volume amounting to some 33%. Through the hy- dration of eruptive olivine-bearing rocks, or rocks rich in other magnesian silicate minerals, have originated a large proportion of the so-called serpentines and verd-antique marbles. Many serpentines and serpentinous limestones are derived from meta-. Please no
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectpetrolo, bookyear1913
