98 REPORTS ON INVESTIGATIONS AND PROJECTS. 



heated to 575 °, and therefore differs from quartz, but it goes over into tridy- 

 mite like ^-quartz at 870 , after which its behavior is that of normal tridy- 

 mite. The conditions of its formation are not yet known. 



All these forms of silica are readily identified, and some at least leave a 

 more or less perfect record of the changes through which they have passed. 

 They are thus of considerable assistance to geologists in determining the suc- 

 cession of temperatures in given geologic processes. 



Portland Cement.— Next to silica in point of seniority is the investigation 

 of the conditions of association of the three oxides — lime, alumina, and 

 silica — three of the commonest components of all igneous rocks and inci- 

 dentally the three chief ingredients of artificial building-stone (Portland 



cement). 



The purpose of these studies, whether considered in relation to natural 

 rocks or to artificial cements, was to ascertain : 



(1) All the compounds of these three ingredients which are possible. 



(2) The temperatures within which each of these compounds is 



stable and therefore capable of independent existence. 



(3) The relation between each compound and any or all of the others 



at whatever temperature. 



(4) The behavior of these individual compounds or groups of them 



in the presence of water at various temperatures. 



(5) The application of this information to the study of natural rocks 



or to the making of artificial rocks (cements). 



The first three of these studies include the major portion of the labor and 

 are now practically completed, (1) and (3) are in fact already published, 

 (2) will appear in print early in the coming year, and (4) is now being ac- 

 tively prosecuted; (5) is not altogether within the province of the labora- 

 tory, but it is our purpose, at the close of the investigation, to indicate the 

 more important applications of the information obtained which chance to be 

 within our experience. 



Mineral Sulphides. — To the paper of a year or more ago on the mineral 

 sulphides of iron has now been added a second contribution to the elucida- 

 tion of the general question of the conditions of formation of the sulphide 

 ore-bodies which contribute so largely to our mineral wealth. The study of 

 the sulphides of iron served to clear up the long-mooted question of the true 

 composition of and relation between pyrite, marcasite, and pyrrhotite and the 

 conditions favorable to their formation. The paper on "The Sulphides of 

 Zinc, Cadmium, and Mercury, their Crystalline Forms and Genetic Con- 

 ditions" (reviewed on p. 106) extends the inquiry to three new groups of 

 sulphides and reaches equally definite conclusions. It is now proposed to 

 continue on to the sulphides of copper and eventually to those of lead. It is 

 probable that in these latter groups the relations are somewhat more com- 

 plicated than those hitherto studied, but their importance is very great. 



