Abstract

Introduction

In the summer of 1927 the writers found numerous pebbles and boulders of crystalline rock in firm fossiliferous limestone, of upper Pennsylvanian age, in La Salle County, Illinois. This limestone is in the upper part of the McLeansboro formation, here about 55 to 60 feet above the La Salle limestone and doubtless of the same age as part of the Conemaugh division, in the Appalachian region. Most of the pebbles were obtained from strata exposed along a ravine on the south side of Illinois River, in the northwest ¼, section 27, of La Salle township (township 33 north, range 1 east), about 2 miles southwest of La Salle. Others were found in limestone somewhat lower in the section, exposed about 5 miles farther northwest, near the town of Spring Valley. The pebbles were scattered through the limestone and no pebbles or fragments of sedimentary rock were found in the . . .

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