BY H. I. JENSEN. 529 



instead of finding very great thicknesses of schist of uniform 

 composition there is a constant change; each bed is of only com- 

 paratively slight thickness and extent, and differs sharply from 

 the overlying and underlying beds. 



The above facts are quoted from Griibenmann largely in his 

 own words. 



It is also stated by the same author that all known kinds of 

 sedimentary rock have been found metamorphosed into schist 

 except salt-beds. Salt-deposits are found in Tertiary, Mesozoic 

 and Palaeozoic formations, but are apparently not met with in the 

 metamorphic, Azoic series. This does not necessarily mean that 

 salt-beds were not formed in Azoic times, but it does show that 

 they are incapable of resisting metamorphosis, and are either 

 rendered plastic (like glacier-ice) and flow away, or they chemi- 

 cally attack over- and underlying siliceous rocks and form an 

 alkaline rock. 



In Tables A and B (pp. 530-531) the chemical composition of 

 some alkaline, sodic, sedimentary rocks is recorded, and it can 

 readily be seen that they are closely related to alkaline igneous rock. 



Jadeite, which has the approximate chemical composition of 

 elseolite syenite, is a rock belonging to the deepest zone, and it 

 has, as far as known at present, no meso- and e/n-equivalents. In 

 some localities its associationship suggests sedimentary origin. 



Chloromelanite is a rock which belongs to the lower and middle 

 zone, and its equivalent in the upper zone is glaucophane albite 

 schist. These rocks have close affinities with theralitic and mon- 

 zonitic magmas, and are known in some ca^es to have been 

 derived from alkaline sediments. 



All such rocks might very well have been derived from a 

 chemical combination of alkaline beds, the precipitates of the 

 early ocean, with adjacent rocks, and increase of pressure would 

 tend to squeeze out such magmas in the form of alkaline lavas. 

 The existence of jade only in the lower zone might be due to salt 

 beds which formed later than the primitive ocean period having 

 been of much smaller thickness and extent, and consequently 



