174 A. L. DAY MINERAL RELATIONS FROM LABORATORY VIEAVPOINT 



point, not by one degree or two, but by 10, 30, 50, or 100 degrees, often- 

 times according to the character of the added mineral and the amount of 

 it which is present. 



The adjoining figure (figure 7) representing the complete determina- 

 tion of mixtures of calcium and magnesium metasilicates contains several 

 illustrations of all the features described. 



Adding a third mineral to the first two will not only still further lower 

 the melting temperatures, but will increase the number of phases and 

 eutectics to the point of seriously complicating their interpretation. To 

 disentangle such a group of phenomena from the appearance of the mix- 

 ture in the furnace while the changes are going on is quite out of the 

 question. 



Incomplete Eeactions 



In concluding the discussion of those minerals whose properties are 

 difficult to establish because of their failure to respond promptly to 

 changes of pressure or temperature, attention should be directed to the 

 extreme case, which also is by no means uncommon. If nature has suc- 

 ceeded in crystallizing certain of her most common minerals only through 

 the presence of volatile ingredients, and sometimes not at all (the Ob- 

 sidian Cliffs) when none are present, she has quite as often paused in the 

 transition from less stable to more stable solid forms. Even qualitative 

 experiments with the blowpipe have developed a great number of cases of 

 instability due to incomplete reaction. One has been worked out in de- 

 tail in this laboratory. Two forms of magnesium metasilicate have been 

 found which are more stable than enstatite; one has been identified in the 

 Bishopville meteorite, which is supposed to have encountered a very high 

 temperature; the other has never been found in nature. Even before this 

 relation was established, Yogt advanced the hypothesis with a considerable 

 degree of positiveness that all the natural amphiboles represent unstable 

 forms or the incomplete development of the corresponding p3Toxenes, and 

 there are many cases of this kind. The minerals which have segregated 

 from the magma have come out step by step in one combination or an- 

 other, and in one cr3^stal form or another, in obedience to a definite system 

 of transformations, following definite relations of pressure, temperature, 

 and composition. Such a series of transformations may have reached 

 complete stability at the temperature and pressure now prevailing, or it 

 may not. There is a time factor which obtains in all reactions, and it by 

 no means follows, as we have frequently seen, that because geologic time is 

 reckoned in large intervals, sufficient opportunity has been given for all 

 mineral reactions to become complete. If a mineral appears in excess in 



