266 PKOCEEDIXGS OF SECTION C. 



igneous rocks became known in more and more exact terms, though 

 much remains at present to be learned of the definite chemical com- 

 position of some of the common mineral components of most rocks. 

 Chemical analyses of rocks are becoming more complete, and more 

 frequent in petrographical publications, and the store of chemical data 

 i.^ steadily increasing-, and has been made more available by the collec- 

 tion of rock analyses published by Roth, and more recently by Wash- 

 ington. 



The description of igneous rocks has been largely fortuitous. As 

 rocks happen to have been encountered in geological field work, they 

 were collected, and not always with due regard to their geological 

 relations to other rock bodies, and subsequently they were investigated 

 in the laboratory more or less thoroughly, and described, often very 

 imperfectly. Up to recent times the tenns '"' petrography " and " petro- 

 grapher" applied satisfactorily to the subject and to the worker in it, 

 for the w^ork was chiefly descriptive. 



Generalisations regarding the nature of igneous rocks, or the for- 

 mulation of laws controlling their crystallisation were largely empirical 

 dicta not infi'equently based on incomplete knowledge or inadequate 

 experience. As a natural consequence of the haphazard manner of 

 growth of the science, thei'e has been an unsystematic nomenclature, 

 derived from many sources at widely remote times, expressing 

 njarkedly different degrees of information regarding the thing de- 

 scribed ; rock, texture, or relationship. And in many instances repre- 

 senting in a single term a series of definitions A^arying with shifting 

 opinion or advancing knowledge. Such, for example, as syenite, 

 granite, and trachyte. 



At the present time attempts are being made to apply to the 

 study of igneous rocks the results of laboratoiy experience in physical 

 chemistry and, not only to investigate directly the physical behaviour 

 of molten rock minerals singly and in combinations, or mixtures, but 

 to apply the more advanced laws of physicochemical reactions to the 

 elucidation of the problems of ciystallisation, differentiation, and 

 mineral composition. The researches of Day and his colleagues in the 

 geophysical laboratory of the Carnegie Institution of Washington, 

 D.C., upon temperatures of fusion, ciystallisation, and transformation 

 points of silicate compounds corresponding to rock minerals, and of 

 the behaviour of mixtures of pairs of such compounds in producing 

 mixed ciystals, new compounds, or eutectic mixtures, are of the first 

 importance. The accuracy of the methods employed and the thorough- 

 ness of the work guarantee the value of the results and their per- 

 manency. In addition to the establishment of improved, or entirely 

 new methods of operation of a purely physical character tributary to 

 the study of petrological problems, they have determined the physical 

 behaviour of the lime-soda-feldspar series; the relations of the various 

 lime-silica compounds to one another; those of the lime-magnesia- 

 metasilicate series ; the melting and transition points of quartz and 

 tridymite, and the character of still other compounds. 



Doelter and his pupils have studied the fusibility of the rock 

 minerals and their solubility in one another, but the methods employed 

 are less accurate than those just mentioned, and involve a large 

 element of subjectivity. They are approximations to the facts desired, 



