The shales in the thick Tertiary section of southern Louisiana are largely uniform and lack lithologic contrasts, with the result that conventional logging methods and geophysical surveys often yield inconclusive data. In an effort to provide useful supplementary information, a program of mineralogical study has been conducted for several years at Columbia University on clays from a significant southern Louisiana locality, Caillou Island. Materials available for study have included a few conventional cores, numerous side-wall cores and extensive suites of cuttings. The principal techniques applied have been bulk density measurement by flotation and indentation as an indication of compaction. In addition, core textures have been examined on polished surfaces and mineralogical identifications have been made by means of the X-ray diffractometer, X-ray fluorescence, and optical examination. Likewise optical data have been supplemented by index determinations on fused clays. Clay mineral constituents of the section are almost entirely illite, montmorillonite, and kaolinite, normally in about the same relative amounts, but on the S. flank of the dome an important change in distribution ranges is recognized at the top of a deep shale zone. Purified Caillou Island illite has been pressed to form a series of small briquettes at regular intervals in the range 0-50,000 p.s.i. to provide reference material compacted under known static load. The briquettes have been tested for bulk density and relative indentation numbers, and the resultant curves have been found to show straight-line relationships. These curves have been used as reference lines in examining bulk densities and indentation numbers of well samples in the range 5000-18,000 ft. Abnormal deviation from the reference curves in certain wells is believed to indicate differential domal compaction in excess of compaction from the gravitational component. The distribution of the anomalies may be structurally significant. A deep shale zone S. of the Caillou Island dome yields an unexpected reduction in indentation numbers and bulk densities with some redistribution of clay mineral proportions. The break in bulk density and indentation curves at the top of this zone in a local area suggests low-angle fault movement associated with the domal intrusion.