592 PETROLOGY AND MINERALOGY 



of petrography, are certainly destined to become a part of it through 

 peaceful assimilation, just as every rock used in the construction of 

 a building thereby becomes a building-stone, no matter what other 

 name it may go by. 



We now come to two questions: first, What do the neighboring 

 sciences contribute to the development of petrography ? and second, 

 What does petrography contribute from the range of its own experi- 

 ences toward the understanding of phenomena or the solution of 

 problems belonging to neighboring provinces? In reply to these ques- 

 tions it would appear that on the w r hole our science receives more help 

 than it gives, although it is not nourished and supported by other 

 sciences to the same extent as is that great complex of heterogeneous 

 sciences known as modern geography. 



With respect to the relationship of petrography and mineralogy, 

 however, conditions are quite the opposite. Every one who has been 

 actively engaged along both these lines of study during the past dec- 

 ade, and especially those who, like myself, have developed con- 

 temporaneously with the rapid modern growth of petrography, will 

 admit that purely petrographic studies have been infinitely more 

 fruitful to mineralogy than vice versa. It is true that as early as 

 during the fifties there had been scattered, disconnected attempts to 

 study isolated minerals by means of the microscope; but these at- 

 tempts remained without further significance because of the indiffer- 

 ence, skepticism, and lack of comprehension which then prevailed. 

 General and methodical microscopic studies were first concerned 

 with the thin sections of those minerals important as being constit- 

 uents of rock-species, and whose recognition was, therefore, one of 

 the chief problems of petrology (Gesteinskunde) . Thus all these in- 

 terpretations were undertaken rather in the service of petrography 

 than of mineralogy. All those peculiarities of the rock-forming 

 minerals which the petrologist was thus determining and studying 

 with ever-increasing zeal, -- the positions of their optic and elasticity 

 axes, their coefficients of refraction and of absorption, their relative 

 cohesive strengths, their twinning laws, and their finer structural 

 conditions, the nature of their solid and fluid inclusions, the phe- 

 nomena of alteration and weathering, their reconstruction into new 

 epigenetic substances, all this knowledge has been contributed to 

 mineralogy proper. It was not until the necessity arose for study- 

 ing the petrographic associations of many minerals that we obtained 

 light on the history of their development. Until petrology included 

 them in its province, how meager was our knowledge of titanite, silli- 

 manite, cordierite, zoisite, tridymite, nepheline, leucite, mellilite, 

 and many of the feldspars, of the members of the pyroxene-amphibole 

 group! How poor the text-books on mineralogy would appear, if all 

 of that material based on petrographic work, which now enriches and 



