SEDIMENTARY ROCKS/Mineralogy and Classification 29 Figure Photomicrograph of arkose under polarized light Torridonian, Precambrian, Scotland Note the abundance of twinned feldspar and the better sorted texture than in Figures and Reproduced with permission from Selley RC (2000) Applied Sedimentol ogy, 2nd edn London: Academic Press Figure Photomicrograph of a quartz arenite under ordinary light Simpson Group, Ordovician, Oklahoma, USA Note the well sorted texture The framework grains are almost entirely composed of well rounded and well sorted quartz There is neither matrix nor cement Reproduced with permission from Selley RC (2000) Applied Sedimentology, 2nd edn London: Academic Press composed, by definition, of predominantly silt or clay, respectively They are easy to identify in the field Siltstones have an unpleasant gritty feel between the teeth, clays a pleasing plastic texture When sufficiently indurated, both siltstones and claystones may be termed ‘shales’ The term ‘shale’ was defined by Pettijohn as ‘‘a laminated or fissile claystone or siltstone’’ This is a widely used term, and with good reason: siltstones are commonly argillaceous and claystones silty, and so the term shale covers all variations The term ‘mudrock’ or ‘mudstone’ has been applied to siltstones and claystones (and their admixtures) that not possess the fissility of shale Siltstones are composed of detrital quartz, shell fragments, assorted heavy minerals, and, often, mica, which imparts the fissility to shale Siltstones