Prof. F. Lcewimon-Lessing — Origin of the Igneous Rocks. 295 



When such a refusion, or ' anatexis ' as it is called by Sederholm, 1 

 embraces a portion of the crust, which consists of definite eruptive 

 rocks, e.g. granite, gabbro, basalt, the resulting magma will again 

 be after consolidation the same rock, perhaps only slightly modified 

 by the assimilation of other material during the passage of the magma 

 to the place where it consolidates. But when the re-melted portion 

 of the crust is composed of different rocks, eruptive or sedimentary, 

 or both together, the process is rather a 'syntexis', as I have called 

 it, an assimilation which is followed by liquation and differentiation ; 

 the same process of ' syntexis ' or assimilation must take place at the 

 margins of such re-melted portions where they are in contact with 

 non-melted portions. Although I am an advocate of the hypothesis 

 of a fluid nucleus, I do not believe that this nucleus would have been 

 (perhaps with a few exceptions) the source of the Archaean and post- 

 Arch aean intrusive bodies and superficial volcanic masses ; but certainly 

 it must be admitted that different portions of the solid or anatectic 

 crust can be mixed during the process of refusion with fluid material 

 coming from peripheric magmatic basins. 



Thus the face of the solid crust, as far as we know it, has been from 

 the Archaean up to the present time often modified and renewed by 

 weathering, sedimentation, fusion, and recrystallization of the material 

 that had formed the crust in a remote pre- Archaean period lying 

 beyond the limits of direct investigation. No wonder that in all 

 post-Archaean periods, as well as in the Archaean, we meet always 

 the same granitic, syenitic, and basaltic rocks, the same gabbros, 

 diabases, porphyries, etc. It is always the same material, perhaps 

 receiving sometimes an admixture from below, which leads under the 

 influence of assimilation to differentiation, and with a tendency to 

 eutectics, resulting always in the same types of eruptive rocks. 



The answer to the question put at the beginning of this section 

 would then be the following: The source whence came the igneous 

 rocks, beginning with the Archaean, is the solid earth's crust, different 

 parts of which, periodically re-melted, have been brought to a magmatic 

 state ; these magmas in certain cases correspond directly to further 

 rock-types or are brought to these types by differentiation. Originally 

 the oldest igneous rocks were formed from magmatic masses of granite 

 or gabbroidal composition. Such a conception simplifies our problem, 

 and the identity of rock-types in all periods receives a plausible 

 explanation ; but it would be difficult to understand and to explain 

 why in all periods the same rock-types were always formed, if we 

 were to assume that every eruptive rock is a part of the fluid nucleus 

 or of a peripheric magmatic basin, solidified for the first time where we 

 find it now. 



When I say that we find in the history of the earth a repetition of 

 the same rock-types, I mean the predominating rock-families, as 

 granites, gabbros, pyroxenites, peridotites, syenites, diabases, basalts, 

 porphyries, etc. But, of course, if we take into consideration that 

 the lithological composition of the crust gets more complicated and 



1 J. Sederholm, " Om Granit och Gneiss" : Bull. Comm. Geol. Finlande, 

 No. 23, 1907. 



