The following conclusions concerning this area of about 3,800 square miles in southeastern California, are based upon about 25 months field work between 1921 and 1934. The region records almost uninterrupted sedimentation during Paleozoic and Mesozoic time. Before this, there was a sedimentation (Pahrump series about 5,000 feet thick) and this rested upon a crystalline basement (Archean). The Tertiary record of sedimentation and volcanism appears to be wholly late Miocene or early Pliocene. Two major orogenies are recorded by thrust faults and normal faults. The first (Laramide late Cretaceous or early Tertiary) includes at least five major thrust faults along which early Paleozoic rocks generally rest upon upper Paleozoic or early Mesozoic rocks. Great masses of quartz-m nzonite were intruded toward the close of the epoch and there was widespread mineralization. It was followed by profound erosion from early Eocene to upper Miocene time. The second orogeny (early Pliocene) followed a period of Mid-Tertiary sedimentation and volcanism. It is represented by a single great thrust fault, remnants of the upper plate of which have been mapped over an area of 20 by 30 miles. It was followed by normal faults and local sedimentation.

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