30 PROF. W. C. UEOGGER ON THE [Feb. 1 894, 



belief that the differentiation of the original magma has, to an 

 essential degree, been dependent on the laws which govern the 

 sequence of crystallization, an opinion already maintained by the 

 author several years ago (1886), and farther sustained, chiefly here 

 in England, by Teall. 



The most easily crystallizable compounds (the least soluble) of 

 the magma have in the diffusion first accumulated along the 

 cooling margin. In the liquid magma itself the different com- 

 pounds were probably dissociated, but the degree of dissociation 

 most probably decreased with diminution of temperature and pres- 

 sure. Along the cooling margin of the magma a concentration 

 of dissociated compounds to less dissociated groups has thus taken 

 place, a concentration probably governed by the laws of chemical 

 affinity (see the views of J. H. L. Yogt). These compounds 

 (not identical with the ' Jcerne ' of H. Kosenbusch) concentrated 

 by the diffusion of the less soluble and more easily crystallizable 

 substances, under the stated conditions along the contact-margin 

 of the magma, have then still been liquid or at least been liquid in 

 the main. In the camptonites and the bostonites the consolidation 

 evidently first began after the outburst of the magma into the 

 fissures, which it has filled up. 



The opinion that the most easily crystallizable compounds have 

 diffused to the cooler portion of the magma and there have 

 generated a magma-stratum of peculiar composition has been 

 doubted on the grounds that the rock crystallized after the eruption 

 does not show a stoichiometric composition l ; however, it seems 

 quite probable that this objection is not decisive. That, in the case 

 in question, the camptonite-magma differentiated from the olivine- 

 gabbro-diabase-magma has not exactly the same composition as 

 brown hornblende, finds a natural explanation in the fact that the 

 spaces of crystallization for minerals which can crystallize out of a 

 magma of given composition are well known to partly cover and 

 transgress each other. A diffusion of the compounds of the brown 

 hornblende, to the cooler margin of a magma of the composition 

 above supposed, could not therefore have formed a magma of a pure 

 hornblendic mixture, but only such a composition mixed with an 

 addition of other compounds in subordinate quantities. This 

 admixture is, however, in our case not of any great importance. 

 Then we also find — agreeing with the fact that the felspar in the 

 camptonites crystallized as a rule after the hornblende — that the re- 

 siduary bostonite-magma after the differentiation of the camptonite- 

 magma (setting aside the unimportant percentage of magnesia, iron 

 oxides, and titanic oxide) shows an almost felspathic composition 

 of medium acidity. 



I think I have proved that the camptonites and bostonites in 

 Gran have been differentiated from a magma of the average com- 

 position of the olivine-gabbro-diabases which appear in the same 

 tract. If the above-explained hypothesis be admitted, this fact 

 seems not to be at variance with the observations from other countries, 



1 Iddings, ' On the Origin of Igneous Bocks,' Bull. Phil. Soc. Washington, 

 vol. xii. (1892) pp. 89-214. 



