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SCIENCE 



[N. S. Vol. XXIX. No. 736 



as "plutonie," were still thought of as 

 different from "volcanic" rocks. Some of 

 the commonest were considered as extreme 

 forms of metamorphism, and have been so 

 treated until quite recent times by eminent 

 geologists. 



Not only the geological mode of occur- 

 rence of many of these rocks was unknown, 

 or only partially known, but the inherent, 

 material characters were often matters of 

 conjecture. Before the introduction of the 

 microscope by Sorby, in 1850, the mineral- 

 ogical study was confined to the larger, 

 megascopic crystals, except for the micro- 

 scopical investigation of rock fragments 

 and powder by Cordier in the first decade 

 of the last century. And the early chem- 

 ical analysis of rocks, while adding con- 

 siderably to a knowledge of their com- 

 position as a whole, lacked the complete- 

 ness and accuracy of modern analytical 

 methods, and failed to explain the com- 

 position of the rocks because of the absence 

 of satisfactory knowledge of the mineral 

 components. 



With improved methods of investigation, 

 geological, mineralogical and chemical, 

 knowledge of the character and composi- 

 tion of rocks advanced. The supposed dis- 

 tinction between "volcanic" and "plu- 

 tonie" broke down, or assumed new defini- 

 tion, through the observations and writings 

 of Judd and others. The term "igneous 

 rocks" came into more general use, and 

 embraced all "volcanic" and "plutonie" 

 masses. The mineral composition of all 

 crystallized igneous rocks became known in 

 more and more exact terms, though much 

 remains at present to be learned of the 

 definite chemical composition 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 is steadily increasing and 

 has been made more available by the collec- 



tions of rock analyses published by Roth 

 and more recently by Washington. 



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 

 terms "petrography" and "petrographer" 

 applied satisfactorily to the subject and to 

 the worker in it, for the work was chiefly 

 descriptive. 



Generalizations regarding the nature of 

 igneous rocks, or the formulation of laws 

 controlling their crystallization, were large- 

 ly empirical dicta not infrequently based 

 on incomplete knowledge or inadequate 

 experience. As a natural consequence of 

 the haphazard manner of growth of the 

 science, there has been an unsystematic 

 nomenclature, derived from many sources 

 at widely remote times, expressing mark- 

 edly different degrees of information re- 

 garding the thing described — rock, texture 

 or relationship— and in many instances 

 representing in a single term a series of 

 definitions varying with shifting opinion 

 or advancing knowledge. Such, for ex- 

 ample, as syenite, granite and tracliyte. 



At the present time attempts are being 

 made to apply to the study of igneous rocks 

 the results of laboratory experience in 

 physical chemistry and, not only to investi- 

 gate directly the physical behavior of 

 molten rock minerals singly and in com- 

 binations, or mixtures, but to apply the 

 more advanced laws of physicochemical 

 reactions to the elucidation of the prob- 

 lems of crystallization, 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 



