Dec. J7, 191S Petrography of Some North Carolina Soils 575 



Tourmaline, sillimanite, rutile, and zircon persist in many soil series; 

 in fact, in very few in thia State are they entirely absent. They are 

 extremely resistant in character, which is undoubtedly the cause of their 

 persistence. 



The soils of the Cecil series are by far the most predominating of the 

 Piedmont Plateau. Though formed from the same general character of 

 rocks, they differ decidedly in mineral complexity. The quantity of 

 minerals other than quartz in the Porters series is nearly double that 

 of the Cecil series. However, minerals of nearly the same kind are 

 encountered in both. As a general rule, greater decomposition has 

 taken place among the minerals of the Piedmont soils ; especially is this 

 true of the silt particles. In many of the clay types of the Cecil soils 

 biotite mica is found in only minute quantities, which would tend to 

 show that it is passing out of existence in these older soils. Plagioclase 

 feldspars and apatite are found only in very minute quantities in the 

 soils of this series. Even the quartz particles appear to have undergone 

 much greater wearing than in the mountains. 



This is in accord with the work of Coffey ^ m showing the effect of 

 topography upon the composition of soils. In the mountains the forces 

 of erosion have not allowed the soil mantle to become as well defined 

 as it is in the Piedmont Plateau; consequently, there is greater pre- 

 ponderance of the minerals found in the parent rocks when the super- 

 ficial covering is removed. This fact is better illustrated in the accom- 

 panying reproductions of photomicrographs of representative soils of 

 the two provinces (PI. LII). Quartz and some of the other minerals are 

 eliminated in these cuts, but the relative number of minerals other than 

 quartz in the two samples is easily discernible. 



The Iredell soils are formed from the basic eruptives, mica diorite, 

 gabbro-diorite, and meta-gabbro. Quartz is a subordinate mineral, for 

 in the sand portions of five samples whose averages were taken 80 per 

 cent of other minerals than quartz is found. Among the silt particles 

 quartz amounts to only about 5 per cent of the total minerals. Epidote, 

 hornblende, and augite compose the greater part of the particles of 

 coarser texture, while biotite and pyioxene are found more abundantly 

 in the silt. Very little decomposition had taken place among any of the 

 minerals found in this series; even the plagioclase feldspars, which occur 

 in rather large quantities, do not show signs of serious chemical decom- 

 position. An interesting point is the scarcity of the potash feldspars, 

 orthoclase and microcline. Apatite is found in much larger proportions 

 than in any other soil series in North Carolina, which is in accord with 

 the total chemical analysis. As an average of five samples of the Iredell 

 loam, the phosphoric-acid content is found to be 6,251 pounds per acre 



iCoffey, G. N. A study of the soils of the United States. U. S. Dept. Agr. Bur. Soils Bui. 8s, 114 p., 

 map. 1912. 



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