The structure of 306 samples of seemingly homogeneous massive sandstone from 74 formations was studied by use of radiography. This technique consists of placing a large thin slice of rock directly upon photographic film and exposing it to an X-ray source from a standard medical or industrial X-ray unit. The image recorded on the emulsion depends on differences in X-ray absorption by the various constituents in the rock sample. Density variations between quartz and heavy minerals, clays, and other minor impurities are recorded on the radiograph and clearly outline the internal structure of the rock.

The results of this study show that sandstones that seem to be homogeneous, massive, and completely structureless in outcrop and hand specimen actually contain a definite systematic arrangement of grains into small structural units. These units may be horizontal laminae, cross-laminae, micro-cross-laminae, disrupted bedding, or other types of stratification. A massive bed may contain only one structural type or several types in various combinations. The type of structure present seems to be related to grain size. Micro-cross-laminae and disrupted bedding are most common in the fine-grain deposits, whereas large-scale cross-laminae are restricted to coarser sediments.

It is concluded that massive sandstones do not represent special environmental conditions but were formed by the same processes that produce well stratified deposits. On the basis of this study it is doubtful that any sandstones are completely structureless and isotropic throughout.

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