640 CHARLES GORDON CARLSON 



decomposition. The latter seems the more reasonable conclusion 

 since many of the sediments studied have had their origin in areas 

 of basic igneous rocks. At Keweenaw Point for example the 

 Keweenawan sediments show a very small amount of the basic 

 feldspars as compared to the acid varieties and yet the sediments 

 have been largely derived from rocks of a decidedly basic character 

 and from rocks in which basic feldspars are known to be very 

 common. It is also generally recognized that the calcic feldspars 

 are more readily decomposed than the more alkaline varieties. 

 Iddings states that 



The alkalcic feldspars are not attacked by hydrochloric acid. The more 

 calcic feldspars are decomposed by the acid in proportion to their content of 

 calcium. Thus oligoclase and andesine are not attacked, labradorite is slightly 

 acted upon, bytownite and anorthite are decomposed with the separation of 

 gelatinous sihca. In the rocks the more calcic feldspars are more readily 

 decomposed than the more alkalcic feldspars in general.^ 



Feldspars are much more common in sediments than has 

 generally been supposed. A large number of "sandstones" and 

 "quartz" sands were in many cases found to have a considerable 

 percentage of feldspar. Sands with a 5 per cent content of feldspar 

 are not at all uncommon, while certain glacial and marine beach 

 sands may contain feldspar up to 25 per cent. 



6. Feldspar range of rocks studied. — -It was desired to determine, 

 by the work pursued in connection with this thesis, just what 

 range in feldspar composition can be expected in sediments, and 

 further to ascertain whether, during anamorphism, there is any 

 change in the feldspar composition of the original igneous or sedi- 

 mentary equivalent. The results obtained show that almost any 

 combination of the various feldspars can be found in sedimentary 

 rocks. Of the twenty-four samples studied, these samples includ- 

 ing unconsolidated and consolidated sediments, twenty-three 

 showed a range in feldspar composition from albite to andesine. 

 Labradorite was found in eleven of the samples, while anorthite, 

 due undoubtedly largely to its ready solubility as well as compara- 

 tive rarity was not noted in any of the samples studied. As was 

 expected glacial and marine beach sands show a very large range 



^ J. P. Iddings, Rock Minerals, p. 204. 



