254 Prof. F. Loewinson-Lessing — Origin of the Igneous Rocks. 



is at present difficult, indeed a satisfactory solution of the problem 

 is probably not yet possible. We must remember that to the 

 Silurian and Devonian diabases and melaphyres, we can oppose the 

 Carboniferous and Permian porphyries, and that in the Archaean 

 and Algonkian formations the porphyries, keratophyres, and por- 

 phyrides have an equal or even a larger development than the basic 

 eruptions. The Tertiary fissure eruptions belong of course to the 

 basaltic formations; but we know so little of the Jurassic and 

 Cretaceous eruptions, and what would they tell us ? We must not 

 forget the trachytic, dacitic, and liparitic eruptions of the Tertiary, 

 and we must also take into consideration that the basalts and andesites 

 are not always the last products of eruption. If we also remember 

 the before-mentioned quantitative relations between the granitic and 

 the basic intrusives, we must confess that there are not sufficient data 

 for considering the basaltic magma as the most widely developed one, 

 and as representing the primordial and average terrestrial magma. 



If we summarize all that has been said hitherto, we arrive at the 

 conclusion that the average composition of the terrestrial magma does 

 not correspond to a syenite, nor to a monzonite or essexite, a diorite, 

 a granite, or a basalt (gabbro). Moreover, every hypothesis which 

 regards the average terrestrial magma as corresponding to one definite 

 eruptive rock and assumes only one primordial magma, from which 

 all eruptive rocks are to be deduced, is erroneous ; such a hypothesis 

 could hardly be made to explain in a satisfactory manner all the 

 peculiarities of distribution and the genetic relations of all eruptive 

 rocks. 



The mutual relations of the eruptive rocks may be explained most 

 satisfactorily by the admission of two primordial magmas : a granitic 

 and a gabbroidal (basaltic) one. In what way these two magmas 

 have been generated and how they are distributed in the earth's crust, 

 is a question that may remain open meanwhile. We take it as a fact 

 that there must be admitted two independent primordial magmas, and 

 only these two magmas, while all other eruptive rocks are derivates 

 from them, originating by assimilation and differentiation. Such 

 a hypothesis is based on the following facts and considerations. 



1. Two primordial magmas of granitoidal and gabbroidal composition, 

 entering into the composition of the solid crust nearly in equal 

 quantities, give just the average composition of this crust which 

 has been found hitherto by the calculations of different authors. 



2. The different members of the granitic formation and the gabbro- 

 pyroxenitic-peridotitic formation occur in much larger bodies and have 

 a far greater development than the other eruptives. We know very 

 large gabbro-noritic intrusive bodies which are often accompanied by 

 a more or less varied series of differentiation products (pyroxenites, 

 peridotites, anorthosites, and different vein- and dyke-rocks), but are 

 quite independent and not connected with other intrusives : I can cite, 

 for instance, the gabbro -formation of the Ural Mountains. We know 

 also in several countries extensive and independent granitic bodies. 

 On the other hand, the other rock-types form far less extensive bodies, 

 have a smaller development, and occur generally as marginal or other 

 facies of the granitic or gabbroidal formation. Thus the syenites are 



