194 A TREATISE ON METAMORPHISM. 



next to them stand the carbonates, and the titanates and phosphates are 

 subordinate. Therefore the acids of the silicates and carbonates — i. e., 

 silicic and carbonic acids — are the great rock-forming acids. 



The natural combinations of the elements, so far as they occur as 

 important rock-making constituents, their systems of ciystallization, chem- 

 ical formula?, molecular weights, logarithms of molecular weights, specific 

 gravities, logarithms of specific gravities, molecular volumes (i. e., molecular 

 weights divided by the specific gravities), and logarithms of molecular 

 volumes, are given in tables below alphabetically ai-ranged. 



In the tables the chemical formulae are usually those of the smallest 

 possible molecules. There is no attempt to make the formulae correspond 

 with the real molecular structure of the minerals, since in the present state 

 of knowledge it is quite impossible to do this. It therefore follows that 

 the molecular weights, molecular volumes, and logarithms of the same in a 

 given case are relative. The table should not be used to compare the 

 absolute molecular weights and molecular volumes of the different minerals, 

 as this would give wholly misleading results. For instance, actinolite, the 

 second mineral in the table, varies in the amount of magnesium and iron 

 so that it may have three special formula?. These formula? as written 

 make the molecular weight of the mean molecule twice that of the 

 smallest and that of the largest molecule four times that of the smallest. 

 This results in similar variations in the molecular volumes and also consid- 

 erable variations in the logarithms Manifestly there are no such differ- 

 ences as these. Probably the true molecular weights and molecular 

 volumes, and consequently the logarithms, are very close to one another. 

 However, the numbers given in the table serve the purpose, as explained 

 on pages 208-210, of calculating- the changes of volumes when the particulai 

 varieties of the mineral actinolite represented by the formula? are trans- 

 formed to other minerals. These remarks as between the different varieties 

 of actinolite apply equally well as between other minerals and their different 

 varieties. 



The letters D, Gr, H, and C, following formula? or specific gravities, 

 signify that the authorities from which the same are taken are, Dana, 

 Groth, Hintze, and Clarke, respectively. 



