Composition of Rocks and Soils

The igneous rocks of the Serre are mostly granodiorites. The majorityof the metamorphic rocks belong to the transition from greenschist to amphibolites facies, sometimes retrograde. Among the sediments the conglomerates are a distinct but controllable source of misleading results due to recycled gravels. The sandstones are mainly arkosic. The siltstone or olisthostrome complex contains only 20% sand (40% silt, 40% clay). The soils are dystric and eutric cambisols, orthic luvisols, calcaric regosols; we measureda mean of 71% sand, 19% silt and only 10% clay. Soils of granite and feldspathic sandstone show an arkosic composition (F/Rf and F/L> 1), while the samples from gneiss and schist show a lithic composition (F/Rf and F/L< 1). ZTR values of soil vary greatly; soils from metamorphic rocks contain 12% of ZTR and from granitic or sedimentary ones only 4%.