This report discusses the results of the second geological investigation at Chowilla, based on recommendations proposed in the preliminary text. Supplementary drilling has in no way altered the general geological concepts previously expressed. Amendments have been included as a revised set of geological cross sections and an appendix of descriptive bore logs. Experiments indicate that the total water leakage beneath the dam will be considerably less than previously estimated, with maximum and minimum values within much closer limits. Leakage beneath the western abutment will be less than earlier apparent, due to the deeper succession of impermeable sediments revealed by boring. In some areas, sediments immediately underlying the blanket cover will have specific permeability somewhat greater than the average, but generally this will not be so. The probability of piping through these upper materials should thus be substantially reduced, even without mechanical compaction.