Sandstones of the Tunnel City Group, Cambrian, cropping out in Western Wisconsin, contain beds rich in grains of feldspar of generally euhedral outline. Each grain consists of a rounded core. regarded as a detrital fragment from a crystalline provenance, surrounded by a neoformed part of up to 15 % of the grain volume. Analyses of the cores show them to be unzoned, non-perthitic, maximum microclines of remarkably uniform composition averaging Or 93 Ab 6 Cn 1 . Such a composition of low sodium in the cores, as well as in the associated minerals, suggest alteration of feldspars to have occurred before final deposition and certainly preceding crystallization of overgrowths. Overgrowths virtually are endmember potassium feldspar structurally intermediate between adularia and orthoclase. Their habit is similar to that of orthoclase except that {100} pinacoids are developed rather than the common {110} prisms. Various ridges of wedge-like cross section cover the surfaces, and characterize {101}. Such ridges are interpreted as growth interference of new {100} and {001} faces when normal development of {101} was terminated by some unknown event. Physical conditions at the time of authigenesis are regarded as being near 25 degrees C, and pore pressures considerably lower than 100 atm. Overgrowths on feldspars preceded dolomite cementation and are believed to have crystallized from pore waters which initially were of seawater composition. Later these solutions were enriched in alumina, potassium and silica by a process suggested as partial dissolution of unstable detrital feldspars. The concentration increased until supersaturation with respect to microcline prevailed. From this solution a potassium feldspar structurally intermediate between adularia and orthoclase was crystallized on the nuclei of microcline grains.

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