A photogeological test study of an area of about 1100 sq. mi. in the Middle Amazon Basin revealed evidence of structure in spite of low relief and heavy forest cover. The study suggests that a thin surficial formation rests with angular unconformity on a thick northward-dipping sedimentary sequence and that undulations in the mantle formation are structural warps. Active deformation or differential compaction of sediments over buried topographic highs arc believed to have caused these warps. Recent doming by igneous intrusion is not likely because the affected rocks are considerably younger than the last recorded episode of intrusion. The warping has nowhere caused a reversal of dip of the lower sediments but has affected the direction and magnitude of dip.

Ironically, the rivers, which provide the easiest access to much of the interior and along which both field and geophysical surveys are run, may actually have been shunted off structure by the warping. Restriction of exploration to available routes of access, therefore, may be self-defeating as far as structural appraisal of at least some parts of the Amazon Basin is concerned.

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