Very few parent materials associated with soils are entirely uniform in their composition or structure. Frequently, there is some degree of irregularity including foliation, veining, jointing, or layering that in some cases helps with soil formation, and in other cases hampers it. For example, some sedimentary layering promotes the formation of soil such as a silty cover on bedrock, or a sandy cover on a clayey alluvium layer. In both of these cases, a friable surface material has been established by nonpedogenic instances. Other instances of sedimentary surface cementation, or fine interbedded sequences of clay and sand, could be considered to be not conducive to the formation of a soil. Nonuniform parent materials may be difficult to find in soils and paleosols, although deviations from normally found minerals could lend clues to the original parent material. If grains of primary materials are not found in the parent material, it can be inferred that later additions occurred. For example, quartz is not found in basaltic phonolite, and olivine is not found in granite.