Febetjaby 5, 1909] 



SCIENCE 



217 



prolonged period of complex volcanic 

 eruptions. 



Independence of action at neighboring 

 volcanoes, either as to period of eruption, 

 volume of magma erupted, explosive or 

 quiet character of action, or relative height 

 of lava column in conduit of volcanoes, 

 follows from local variation in the factors 

 entering into the process of magma erup- 

 tion, such as, the volume of magma in- 

 volved in each conduit extending to pro- 

 found depths, the shape of the conduit, 

 the temperature of the magma ; the rate of 

 cooling ; the amount of gas diffused in any- 

 given time ; the character of the surround- 

 ing rods; and the stability of the sur- 

 rounding rock masses as a complex whole. 

 The chemical composition of the magma is 

 also a factor involved in the activity of a 

 particular volcano. But the composition 

 of the magma is also a feature by which 

 volcanoes may show independence. Dif- 

 ferences in the composition of rocks in 

 neighboring volcanoes is to be sought in 

 variation in the differentiation of magmas 

 during the course of eruption from deep- 

 seated to surficial positions. 



Among the results of such differentia- 

 tion may be mentioned the production of 

 complementary rocks, which may occur in 

 rock bodies of various forms. A special 

 case of local differentiation, usually as- 

 sociated immediately with crystallization 

 and solidification of parts of a magma, is 

 the production of contemporaneous veins 

 and pegmatites. When complementary 

 rock magmas are erupted so close to one 

 another in space and time that they come 

 in conjunction while still highly heated, 

 they may diffuse into one another, or 

 blende to such an extent as to yield hybrid 

 rocks, or mixed dikes, sheets, etc., as ob- 

 served by Harker on the Isle of Skye. 



The eruption of rock magmas through 

 solid rocks and their solidification in vari- 

 ous positions within or upon other rocks 

 condition the modes of occurrence of igne- 



ous rocks, as those of lava streams, dikes, 

 sheets, laccoliths, etc. And the parting or 

 cracking of the solid rock, upon cooling, or 

 its arrangement after fragmentation in 

 various ways leads to distinctive struc- 

 tures, such as columnar, spheroidal or 

 brecciated. 



Having acquired a knowledge of the 

 general principles applicable to all igneous 

 rocks, it is in order to consider more 

 specifically those occurring in all known, 

 parts of the world : first, systematically, ac- 

 cording to some comprehensive scheme of 

 arrangement, or classification, and then ac- 

 cording to the groups, or associations, in? 

 which they occur in various regions, that: 

 is, according to petrographical provinceSj. 

 or co-magmatic regions. 



In order to describe many rocks a no- 

 menclature is necessary, and the confusion 

 existing in that in present use is best 

 understood by considering the history of 

 the growth of petrography, and the 

 changes that have gone on in the definition 

 and use of the oldest and commonest rock 

 names, and descriptive terms. With this 

 review should be associated a sketch of the 

 development of rock classification, which 

 has been furnished to the student in an 

 interesting form by Cross. 



A successful treatment of the subject of 

 igneous rocks along the lines indicated 

 would go far toward the removal of petrol- 

 ogy from a state of distracting empiricism, 

 and the placing of it on a more rational 

 foundation. Joseph P. Iddings 



Washington, D. C. 



CONDUCT OF SCIENTIFIC WORK UNDER 



THE UNITED STATES GOVERNMENT^ 

 To the Senate and House of Representatives: 

 In compliance with the provisions of section 

 8 of the act of Congress making appropriations 



"Message from the President of the United 

 States, transmitting a report of the National 

 Academy of Sciences relating to the conduct of 

 the scientific work under the United States gov- 

 ernment. 



