36 PKOE. W. C. BKOGGER ON THE [Feb. 1 894, 



the physical conditions under which the differentiation takes place. 

 The ' hern ' hypothesis of Bosenbusch, in the form given by that 

 author, does not fit in with these deductions. 



(4) The observations here described probably further show the 

 inverse case, that the same group of differentiated roclcs (in the 

 present case, caruptonites and bostonites) can be produced by separa- 

 tion from mother-magmas of quite different chemical composition. In 

 our case these dyke-rocks have been derived from an olivine-gabbro- 

 diabase magma : in other instances we know that the same rocks 

 are connected with nepheline-syenites (augite-syenites?), and have 

 probably in those cases been differentiated out of a nepheline- 

 syenitic (augite-syenitic ?) magma. This being so, we have another 

 fact which does not agree with the ' hern ' hypothesis. 



(5) I have endeavoured to prove that the inferred differentiation 

 has been determined by, and is dependent on, the laws of crystalli- 

 zation in a magma in so far as the compounds, which on given con- 

 ditions would first crystallize out of the magma, must have diffused 

 to the cooling margin, and in this way have produced, in the 

 contact-stratum, a peculiar chemical composition in the still liquid 

 magma before any crystallization took place. 



By the pressing-up in this manner of the differentiated masses to 

 a higher level at lower temperature and pressure, or by continued 

 cooling along the contact-margin with subsequent crystallization, 

 there have then been produced different kinds of rock of peculiar 

 chemical composition, miueralogical constitution, and structure. 

 The sequence of eruptions from a common magma-basin (magma- 

 reservoir) must therefore, to a certain extent, be parallel with 

 the sequence of crystallization in the corresponding kinds of 

 rock. Many years ago, I tried to prove that this opinion is 

 really confirmed by my observations on the rock-succession in 

 the Christiania region, which closely conforms to the sequence of 

 crystallization in the corresponding abj'ssal rocks in this ' eruptive 

 province.' A series of confirmatory examples is known from the 

 literature of other countries, from the British Islands by the 

 excellent publications of Sir Archibald Geikie, Messrs. Teall, 

 Dakyns, and other authors. The examples are really so numerous 

 that they seem to represent a general law. No doubt in many cases 

 the rock- sequence in different ' eruptive tracts ' does not appear to 

 agree with this empirically deduced law. But that does not prove 

 that the law has no existence ; it only shows that other conditions 

 besides those above-mentioned are of importance for the determi- 

 nation of the eruptive sequence. A discussion of the great number 

 of different possible cases (for instance, the hypothesis of Iddings 

 as to the eruptive sequence, etc.) would on this occasion lead us too 

 far. Here my intention has been simply to present a series of ob- 

 servations from a single locality, in which the genetic relations 

 between the different kinds of rock seem distinctly to favour the 

 opinion of a conformity between the sequence of crystallization and 

 that of differentiation. 



