CALCULATION OF VOLUME RELATIONS. 211 



alterations have been expressed by chemical equations, and in fewer still 

 have the volume relations been calculated. Second, the imperfect 

 equations herein contained will be sure to lead to closer investigations of 

 the nature of the alterations, and to improved equations representing them. 

 Thus the progress of science will be promoted by the set of equations here 

 given, even if the great majority of them are defective. Third, it is 

 believed that when a more nearly correct set of equations is written it will 

 be found that the large majority of the equations herein contained substan- 

 tially represent the facts, and consequently that the volume changes are in 

 most cases roughly approximate. Many of them may be changed by a 

 few per cent one way or the other; but the sign of few will be changed, 

 and this is the fundamental point in reference to the zones in which the 

 alterations occur. 



The weakest point in the accuracy of the volume reactions is not tound 

 in the chemical equations, but in the inexactness of the specific gravities of 

 the minerals as given in the text-books. For most minerals there is a con- 

 siderable range of specific gravity given; and with the exception of one or 

 two minerals, such as calcite and quartz, it is impossible to ascertain the 

 exact specific gravity of the pure minerals. In the table the mean between 

 the two best determined extremes is given as the best approximation 

 available of the specific gravities of the pure minerals. For most minerals 

 these extremes are taken from Dana's System of Mineralogy. 



The facts as to the occurrences and alterations of the various minerals 

 given in the following pages are largely taken from the standard text- 

 books of mineralogy and petrology, and especially from Dana's great 

 System of Mineralogy. The information available is especially imperfect 

 as to the manner in which the complex minerals, and particularly the 

 complex silicates, break up into simpler compounds in the belt of weather- 

 ing. As explained fully in the following chapter, this is a general process. 

 For the better known of these changes equations are written, but no 

 attempt is made to express by equations the manner in which many of the 

 minerals decompose and degenerate, because so little exact information is 

 available upon which to base such equations. 



As already stated, only those minerals will be considered which are 

 important rock-making constituents. It is impracticable at the present time, 

 to consider the physical-chemistry of the rarer minerals. 



