Dec. 27, 1915 
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 1 in 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 pyroxene 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 
1 Coffey, G. N. A study of the soils of the United States. U. S. Dept. Agr. Bur. Soils Bui. 85, 114 p. # 
map. 1912. 
12573°—15 - 3 
