Prof. F. Lceicinson-Lessing — Origin of the Igneous Pocks. 297 



Hunt, 1 who considered the plutonic rocks to he due to thermo- 

 metamorphism. 



These fragmentary citations show sufficiently well that at different 

 times different authors relying on different considerations have come to 

 the same conception, namely, that the igneous rocks are to he considered 

 as re-melted parts of the solid crust — a hypothesis which seems to me 

 best fitted to resolve in a satisfactory manner the problem of the genesis 

 of igneous rocks not older than the Archaean. 



Conclusions. 



1. The method hitherto adopted of calculating the average 

 composition of the terrestrial magma must be considered as erroneous 

 in principle. 



2. Nevertheless these calculations give a fairly good result, as it 

 corresponds approximately to the mean between gabbro and granite, 

 if we admit that these two magmas enter into the composition of the 

 external part of the earth's crust nearly in equal quantities. 



3. Two original independent magmas exist which predominate in 

 the composition of the earth's crust, the granitic and the gabbroidal 

 (basaltic); all the other igneous rocks arederivates from these two and 

 are subordinate to them in their occurrence. 



4. Differentiation is produced in two ways: during the crystal- 

 lization, differentiation by crystallization ; and before crystallization, 

 in the liquid magma — magmatic differentiation. 



5. The differentiation by crystallization consists in the sinking 

 or rising of the newly-formed minerals according to their specific 

 gravity, and in the solution in one part of the magma (generally 

 a deeper-seated one) of minerals formed in another part. 



6. Magmatic differentiation consists in the formation of derived 

 magmas (Spaltimgen), and is governed by the tendency to form 

 eutectic and monomineral (or bimineral) magmas. This differentiation 

 is induced by the fusion and assimilation of foreign mineral masses, 

 both igneous and sedimentary. 



7. Differentiation finds its best explanation in the syntectic- 

 liquational hypothesis (fusion, assimilation, differentiation). 



8. All igneous rocks belong to three types : (1) primordial magmas, 

 (2) rocks due to differentiation, (3) rocks produced by mingling of 

 two magmas. 



9. The igneous rocks of all geological periods, presumably from the 

 Archaean, originated principally by the refusion of different parts 

 of the earth's crust. On account of this we meet in successive periods 

 always the same types of rocks. The pre-Archaean igneous rocks 

 (perhaps also a part of those of the Archaean and younger periods) 

 were formed from primordial magmatic masses of granitic and 

 gabbroidal composition. 



Beaulieu s. Mee. 



January, 1911. 



1 Sterry Hunt, "The Origin of the Crystalline Rocks " : Trans. Eoy. Soc. 

 Canada, ii, p. 3, 1884. 



