STUDY OF IGNEOUS ROCKS. 265 



•worked in many places. Building stojies : These comprise freestone 

 (sandstone), trachyte, granite (Enoggera), porphyrites, and sei-pen- 

 tines. Beach sands: All along the coast, the muddy strips excepted, 

 the beach sands contain layers of black sand, which at certain seasons 

 and in certain weathers can be profitably mined for gold, platinum, 

 osmiridium, tin, and rare earths. 



6.— THE STUDY OF IGNEOUS EOCKS. 



By JOSEPH P. IDDINGS, University of Chicago. 



No branch of petrology presents so attractive a field for investiga- 

 tion and study as that concerned with the origin and formation of 

 igneous rocks. The great problems of metamoiphism, that traverse 

 so much of the earth's dpiamic histoiy and involve so many factors 

 common to the problems of igneous rocks, are less alluring because of 

 their gi'eater complexity and less definite character. While much is 

 being done in each of these fields of rock study it is to the former 

 that I wish to call attention at this time. It is interesting to note 

 how the atttiude of the petrographer toward the subject of igneous 

 rocks has changed with increasing knowledge of their composition, 

 and with advancing experience with the fundamental laws of physics 

 and chemistry'. 



Rocks that were considered igneous a century ago were almost 

 wholly tliose known to have poured forth from volcanic craters, and 

 were for the most pai-t compact, aphanitic lavas, often containing 

 porphyritic ciystais — distinctly volcanic rocks. The great number of 

 phanerocrystalline massive rocks were not generally considered as 

 having the same character and origin as A^olcanic rocks — as being 

 igneous. Their formation was explained in different ways by various 

 geologists. And when treated as " plutonic "" were still thought of as 

 different from '' volcanic " rocks. Some of the commonest were con- 

 sidered, as extreme forms of metamorphism, and have been so treated 

 until quite recent times by eminent geologists. 



Not only the geological mode of occurrence 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 Soi'by, in 1(S50, the mineralogical study was 

 confined to the larger, megascopic crystals, except for the microscopical 

 investigation of rock fragments and powder by Cordier in the first 

 decade of the last centuiy. And the early chemical analysis of rocks, 

 while adding considerably to a knowledge of their composition as a 

 whole, lacked the completeness and accuracy of modem analytical 

 methods, and failed to explain the composition of the rocks because of 

 the absence of satisfactory knowledge of the mineral components. 



With improved, methods of investigation, geological, mineralo- 

 gical, and chemical, knowledge of the character and composition of 

 rocks advanced. The supposed distinction between "volcanic" and 

 "plutonic" broke down, or assumed new definition, through the obser- 

 vations and writings of Judd and others. The tenn "igneous rocks" 

 came into more general use, and embraced all " volcanic " and 

 " plutonic " masses. The mineral composition of all ciystallised 



