256 A CENTUEY OF SCIENCE 



by Emerson on the Deerfield dike and its minerals, and 

 on the occurrence of nephelite syenite at Beemersville, 

 N. J. ; to various interesting articles by Cross on lavas 

 from Colorado and the pneumatolytic and other min- 

 erals associated with them; to important papers by 

 Iddings on the rocks of the volcanoes of the Northwest, 

 and those of the Great Basin, to primary quartz in 

 basalt, and the origin of lithophysae; to the results of 

 researches by G. H. Williams on the rocks of the Cort- 

 landt series, and on peridotite near Syracuse, N. Y. ; to 

 papers by Diller on the peridotites of Kentucky, and 

 recent volcanic eruptions in California; to articles by 

 R. D. Irving on the copper-bearing and other rocks of the 

 Lake Superior region, and to Kemp on dikes and other 

 eruptives in southern New York and northern New 

 Jersey. Other publications would greatly extend this 

 list. 



The Petrologic Era. 



As the chief facts regarding rocks, especially igneous 

 rocks, as to their mineral and chemical composition, their 

 structure and texture and the limits within which these 

 are enclosed, became better known; and the relations, 

 which these bear to the associations of rocks and their 

 modes of occurrence, began to be perceived, the science 

 assumed a broader aspect. The perception that rocks 

 were no longer to be regarded merely as interesting 

 assemblages of minerals, but as entities whose charac- 

 ters and associations had a meaning, increased. More 

 and better rock analyses stimulated interest on the 

 chemical side and this and the genesis of their minerals 

 led to a consideration of the magmas and their func- 

 tions in rock-making. The fact that the different kinds 

 of rocks were not scattered indiscriminately, but that 

 different regions exhibited certain groupings with com- 

 mon characters, was noticed. These features led to 

 attempts to classify igneous rocks on different lines from 

 those hitherto employed, and to account for their origin 

 on broad principles. In other words, the descriptive 

 science of petrography merged into the broader one of 

 petrology. No exact time can be set which marks this 

 passage, since the evolution was gradual. Yet for this 



