608 PEOF. J". P. IDD1NGS 0!N" EXTRUSIVE [NOV. 1 896, 



The statement is made that the order of eruption of extrusive 

 rocks is for the most part dependent on relatively secondary differ- 

 entiation. It is also stated that the order of primary differentiation 

 and the succession of primary eruptions of the differentiated magmas 

 from the ' magma-basin ' can be learned only from the study of 

 the relative ages of the great masses of deep-seated rocks. 



Prof. Brogger then comments at some length upon the general 

 order of eruption as set forth by me in the paper on ' The Origin 

 of Igneous Rocks,' namely, that the succession is generally from 

 rocks of intermediate to those of more and more extreme com- 

 position. 1 And he calls attention to the fact that the conclusions 

 were drawn almost entirely from observations upon extrusive rocks. 

 In these cases also he notes numerous exceptions, and moreover 

 points out that the parent magmas (Stamm-magma) may not always 

 be of like composition. These points, too, were mentioned in the 

 paper cited (pp. 178, 183). 



Upon some of the propositions in question Prof. Brogger and I 

 are in perfect accord, in others we seem to be at variance. We 

 agree as to the fact that the differentiation of magmas intruded 

 into the earth's crust from greater depths produces complementary 

 rocks, which may take the form of dykes (Rosenbusch's Ganggesteine) 

 or may reach the surface as lava-streams, etc. ; that the order of 

 eruption of these products may lead to recurrent series, both among 

 intrusive and among extrusive bodies ; that these series, as a whole, 

 would not represent the differentiation that took place in the 

 original parent magma. All parent magmas do not have the same 

 composition. The magmas first derived from them certainly do not 

 have the same composition, consequently the groups of differentia- 

 tion-products belonging to various derived magmas will present 

 distinct series. And of course the intermediate member of one of 

 these series is like the magma from which the series was directly 

 derived. It has not a fixed composition corresponding to one inter- 

 mediate for all known rocks. 



Eruptions from a body of molten magma into outlying or 

 overlying regions may take place before a differentiation of this 

 particular magma sets in, or after it has progressed little or much. 

 The composition of the various portions of magma drawn off will 

 depend on a variety of conditions, which might be postulated at 

 length. If extrusive bodies of rock be considered apart from their 



beziehungen der Ergussgesteine immer mit kritischer Vorsicht benutzt werden, 

 wenn man die DifFerentiationsgesetze der Tiefenmagmen und ihren Einnuss 

 auf die Altersfolge der Eruptionen einer Eruptionsepoche studiren will ; in 

 der obigen kurzen Ubersicht wurden desbalb aucb fast ausscbliesslich bekannte 

 Beispiele der Altersfolge plutonischer Gesteine beriicksicbtigt, wafcrend die 

 Altersfolge der Ergussgesteine als zum grossen Tkeil von relativ sekundaren 

 Differentiationen abhangig, vorlaufig ausser Acbt gelassen wurde.' ( Op. jam 

 cit. pp. 177, 178.) 



1 Iddings, J. P., ' The Origin of Igneous Eocks,' Bull. Phil. Soc. Washington, 

 vol. xii. (1892) p. 145. 



