﻿480 DE. WHITMAN CEOSS OX THE NATT7EAL [Allg. I9IO, 



The positive assertion of limited association enters also into the 

 definition of many of the kinds of ' dyke rocks.' As an example 

 may be cited the statement concerning camptonite :- — 



' Alle Gesteine vom Camptonittypus gelioren als Ganggefolge zu den foyait- 

 ischen und theralithischen Tiefengesteinen und verhalten sich zu Bostoniten, 

 Tinguaiten u.s.w. wie die Minetten, Kersantite, Vogesite. u.s.w. zu den 

 Apliten.' 1 



It is well also to remember that Rosenbusch considers magmas 

 as made up of one or more stoichiometric compounds, called by him 

 Kerne, some of which are mineral molecules, while others are 

 not. Further, it is essential to consider that llosenbusch refers the 

 principal magmas which have developed by differentiation from a 

 homogeneous earth-magma, through the separation of the Kerne, 

 to two contrasting series, the 'granito-dioritic and gabbro-peridotitic' 

 and the ' foyaitic-theralitic' 



Before entering upon the critical discussion of this matter, I wish 

 to repeat a former statement that 



' I must not be understood as failing to appreciate the great advance in our 

 knowledge of the origin of igneous rock varieties and of their structures, and 

 of the genetic relationships of types which lias come within the past few years 

 largely as a result of ... . the theoretical ideas [of magmatic differentiation] 

 lying back of the systematic scheme advocated by Rosenbusch. One may well 

 deny the desirability of the Bike rock group of Rosenbusch and be at the 

 same time an ardent advocate of the [genetic] theories upon which the group 

 was established, and which have little connection with the fact of geological 

 occurrence expressed in the name.' ~ 



The in appropriateness of the term ' dyke rocks,' as used by 

 Rosenbusch, may be passed over here ; nor is the weakness of the 

 ' Kerntheorie ' proper of great significance in itself. The elaborate 

 review of the latter by Brogger reaches an important conclusion, 

 however : namely, that in the theoretical discussion of magmatic 

 differentiation the ' Kerne ' of Rosenbusch must be replaced by mole- 

 cules common in the minerals of igneous rocks, and that the number 

 of these is somewhat greater than that assumed by Rosenbusch for 

 the ' Kerne.' 3 Brogger's discussion seems to me quite convincing, 

 and the conclusion is certainly in harmony with the facts of the 

 rocks and of modern researches on molten solutions. Iddings 

 advocates the same view in the discussion of differentiation, in his 

 recent work on igneous rocks. 



A peculiar feature of the ' dyke rock ' group of Rosenbusch 

 deserving examination here is the asserted exclusive association of 

 certain kinds with particular deep-seated parent magmas, from which 

 alone they could have been derived by magmatic differentiation. 



1 ' Mikroskopische Physiographic : Massige Gesteine ' 4th ed. vol. ii, pt. i 

 (1908) p. 685. 



2 ' The Geological versus the Petrographical Classification of Igneous Rocks ' 

 Journ. Geol. Chicago, vol. vi (1898) pp. 89-90. 



3 ' Die Eruptivgesteine des Kristianiagebietes, III : Das Ganggefolge des 

 Laurdalits ' 1898 (Vidensk. Selsk. Skrifter, I. Mathemat.-Naturv. Klasse. 1897, 

 No. 6) p. 332. 



