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A determination of the relative volumes of the components of a rock may have for its object description, nomenclature, computation of chemical composition, estimation of desirable or undesirable mineral content, or comparison.A quant{tat{re statement of the m{nera]ogica] composition of a rock {s essential to a good description.%Vhile the mineralogical classification o.f rocks is not on a strictly quantitative basis, yet some estimate of the relative proportions of components, {f only a rough one, is neces• sary before a rock can be named.Chemical nomenclature and chemical composition may be more directly obtained from chemical analysis.The commercial importance of determining directly the proportion of a valuable or harmful mineral is self-evident; and comparisons of mineral compositions of rocks may also have considerable economic value.This is especially true in metal mining and in quarrying stone.Owing to the intimate relationship frequently existing between ore deposits and country rocks, a mineralogical comparison of the rocks in a mining district is frequently of value.A good example of a comparative study of this nature is presented by Barrell in his "Microscopical Petrography of the Elkhorn Mining District, Jefferson County, Montana. m The relative quantities of the components of building and ornamental stones affect their properties and render comparative studies desira'ble.A good example of such a study is Dale's "The Chief Commercial Granites of Massachusetts, New Hampshire and Rhode Island."e
Lincoln et al. (1913) studied this question.