634 CHARLES GORDON CARLSON 



3. The materials studied. — In getting material together for 

 study the attempt was made to make this selection one which 

 would most thoroughly test the feldspar method. The mineralogic 

 composition of unconsoHdated and consohdated sediments as well 

 as of metamorphic rocks of known origin was therefore determined. 

 In order that the sediments might represent the breaking down of 

 as many rock formations as possible, they were chosen so as to 

 include a wide geographic and stratigraphic distribution. The 

 aim was also to avoid limiting the material studied to any one 

 particular realm of deposition. Beach sands as well as sands of 

 glacial, eolion, and locustrine origin were therefore chosen. The 

 consohdated sediments examined included arkoses, graywackes, 

 tuffaceous sandstones, and shales. Since the purpose of studying 

 the metamorphic rocks was to determine whether anamorphism 

 causes any changes in the feldspar composition of the original 

 rock, the gneiss and schists were selected which showed different 

 kinds and degrees of change. 



4. Tabulation of results. — ^The table on page 636 shows the results 

 of the feldspar determinations for the various kinds of material 

 studied. 



5. The relative abundance of feldspars in sediments. — The data 

 available are not sufficient to warrant a dogmatic statement as to 

 whether certain feldspars are more abundant in sediments than 

 others. The studies by the writer of a large number of sediments 

 of different origin, as well as of wide geographic and stratigraphic 

 distribution, suggest very strongly, however, that certain feld- 

 spars are very common in sediments, whereas others are quite 

 rare. Orthoclase, microcline, and the acid plagioclases are 

 much more frequently met with in sediments than the basic feld- 

 spars. Microchne seems to be more common than any of the 

 others, so that a careful study of sands which appear to be entirely 

 composed of quartz usually reveals a few grains of this feldspar. 

 By referring to the accompanying diagram (Fig. i) the relative 

 abundance of the various plagioclase feldspars is strikingly brought 

 out. This abundance of the feldspars mentioned indicates either 

 that they are especially common in the rocks from which they 

 were derived or that the basic plagioclases suffer much more rapid 



