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 the original augite, while the outer dot- 

 ted portion with cleavage lines cutting at sharp and obtuse angles 

 is the secondary hornblende. This change is due to slow and 



^Microscopic Physiography of Roek-making Minerals, Wiley & Son, New 

 York. See also Professor A. Harker 's Petrology for Students. 



