528 THE DISTRIBUTION, ETC., OF ALKALINK ROCKS, 



chemical composition. Certain chemical constituents always 

 present in igneous rocks within de6nite limits may be present in 

 proportions greatly exceeding or much below those limits. This 

 is accepted as an indication of selective weathering and arrange- 

 ment by means of water, The same feature is a characteristic of 

 alkaline igneous rocks. 



The crystalline schists of the deepest zone contain many 

 minerals in common with alkaline rocks, e.g., albite, orthoclase, 

 microcline, microperthite, cordierite, spinel, garnet, sodalite, 

 nepheline, etc., etc. This similarity is, however, to be mainly 

 attributed to hydato-igneous action, or hydrous fusion, under con- 

 siderable pressure in both cases. The abundance of microcline- 

 microperthite phenocrysts in alkaline rocks does, however, show- 

 that the magma rose from the deeper parts of the lower and 

 middle zones where orthoclase and albite can crystallise together 

 in the one molecule. 



Crystalline schists occur at the base of all sedimentary rocks 

 and as the central mass of mountain-chains. All the oldest 

 schists carry the imprints of having originated in the deepest 

 zone; the younger schists have the characteristics of the upper 

 and middle zone. The absence of fossils in these rocks is to some 

 extent compensated for by the wonderful uniformity in the 

 characters of the oldest basal Archaean formations. The sequence 

 is everywhere similar. Even the divisions of the Algonkian 

 display such uniformity that each can be recognised and identified 

 in regions remote from one another. The deepest zones, the 

 katarchseic of Finland, are, according to Sederholm and Frosterus, 

 formed of red granite and show no trace of sedimentary origin. 

 Similarly in America the sedimentary rocks pass, with depth, more 

 and more into gneisses. Sederholm, Griibenmann and many 

 other workers on the schists believe that some of these basal 

 formations of gneiss, granite and schist are part of the original 

 consolidation-crust of the earth. The uniformity of character 

 over the whole earth observed in the.-,e rocks is particularly 

 suggestive of this belief. In the younger mountain-chains where 

 schists have been produced by contact with huge laccoli tic masses, 



