Geology. 157 



13. On Granite and Gneiss, their Origin, Relations and 

 Occurrence in the Pre-cambrian Complex of Fenno- Scandia / 

 by J. J. Sedebholm (Bull. Com. Geol. de Finlande, No. 23, pp. 

 110, pis. 8, map and figs. Helsingfors, 1907.) — The author shows 

 in this paper that the rocks of the Fenno-Scandian shield are 

 divisible into definite groups. Some of these are clearly clastic 

 rocks and agree in character in a general way with the Algonkian 

 of North America ; in other cases, however, where these rocks 

 have been invaded and injected with granites, they are held to 

 have the characters of the Archean of North America as that 

 term is now used. The explanation of the origin of the gneissose 

 rocks is regarded as necessary to solve the riddle of the Archean, 

 not only because the gneisses form so important a part of the 

 Archean complex of the region, but because the granitization is 

 the process which has done most to obliterate the structures of 

 the Archean rocks and their stratigraphical relations to each 

 other. The Finnish gneisses are not dynamically metamorphosed 

 granites but non-homogeneous distinctly veined rocks. The 

 body of the work is devoted to the detailed study and recording 

 of the facts observed on the splendid exposures along a portion 

 of the southern coast of Finland, and from these observations the 

 author draws some important conclusions. Thus he observes 

 gradual passages between granites and mica schists in which 

 no line of demarcation can be drawn. He believes that the 

 strongly contorted structure characteristic of most Finnish 

 gneisses is not a secondary phenomenon in a true sense but that 

 it originated when the rock was in a melting condition. He 

 regards the foliation of the granites, when not of dynamic meta- 

 morphic origin, as formed by the incomplete melting and recrys- 

 tallization of schistose rocks. 



The author holds that in some regions he has found that the 

 basement complexes of the typical Archean sedimentary forma- 

 tions have been preserved though much altered. But in the 

 coast region the phenomena of refusion or resolution have 

 occurred so extensively that it must be assumed that the whole 

 area has been in a melting condition when it was once sunk to so 

 great a depth that it approached the magma ocean or tectosphere 

 of the earth. His ideas concerning the interior of the earth are 

 based on the views of Arrhenius. The magmas under pressure 

 make their way upward through the weakest parts of the crust, 

 dissolving them in large measure. It is assumed that this process 

 of regional resolution does not alter the composition of the 

 magma as a whole because it assimilates almost all of the mate- 

 rial resulting from the destruction of magmatic rocks, whose 

 composition is thus restored. Granitic rocks predominate in the 

 basal complexes which the magma penetrates on its upward way 

 and the sheet of sediments is too thin in comparison to have any 

 serious effect in changing the composition of the great magma 

 masses. 



Am. Jour. Sci. — Fourth Series, Vol. XXV, No. 146.— February, 1908. 

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