. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. 86 PHYSICAL AND CHEMICAL PBOPEETIES OE EOCKS would be out of place here, since it involves the polarization of light and other subjects which must be studied elsewhere. The reader is referred to any authoritative work on the subject of light, and to Professor J. P. Iddings's translation of Professor Rosenbuseh's work on optical mineralogy.^ The method of study is of value, not merely as an aid in determining the mineralogieal composition of a rock, but also, and what is often of more importance, its structure and the various
. A treatise on rocks, rock-weathering and soils;. Petrology; Soils. 86 PHYSICAL AND CHEMICAL PBOPEETIES OE EOCKS would be out of place here, since it involves the polarization of light and other subjects which must be studied elsewhere. The reader is referred to any authoritative work on the subject of light, and to Professor J. P. Iddings's translation of Professor Rosenbuseh's work on optical mineralogy.^ The method of study is of value, not merely as an aid in determining the mineralogieal composition of a rock, but also, and what is often of more importance, its structure and the various changes which have taken place in it since its first consolidation. Rocks are not the definite and unchangeable mineral compounds they were once considered to be, but are rather ever-varying aggregates of minerals, which even in them- selves undergo structural and chemical changes almost without number. It is a common matter to find rock masses which may have had originally the mineral composition and structure of diabase, but which now are mere aggregates of secondary prod- ucts, such as chlorite, epidote, iron oxides, and kaolin, with perhaps scarcely a trace of the unaltered original constituents; yet the rock mass retains its geological identity, and to the naked eye shows little, if any, sign of the changes that have gone on. These and other changes are in part chemical and in part structural or molecular. A very common mineral transformation in basic rocks is that from augite to hornblende. This takes place merely through a molec- ular readjustment of the particles, where- by the augite, with its gray or brown col- ors and rectangular cleavages, passes by ^Ttere^tSrnWendf "^''^itie stages over into a green horn- blende, a mineral of the same chemical composition, but of different crystallographic form. The trans- formation i; an incomplete state is shown in the accompanying figure, in which the central, nearly colorless portion with rectan- gular cleavage represents
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectpetrolo, bookyear1913
