Sixty-two samples from well-established comagmatic granitoid sequences and certain unassigned formations and plutons of the central part of the Sierra Nevada batholith between latitudes 37° and 38° N. have been dated by the isotopic U-Pb method on zircon. The U-Pb ages indicate the following age distribution of the granitoids: (1) The axial part of the batholith is occupied by Cretaceous granitoid sequences that are progressively younger eastward over a 37-m.y. interval extending from about 125 m.y. to about 88 m.y. ago. (2) A single, but extensive, Triassic sequence with an optimum average age of about 210 m.y. is present in the east side of the batholith. (3) Plutons and granitoid sequences of Jurassic age, most of them with U-Pb ages between 186 and 155 m.y., occur in both margins and locally in the interior of the batholith. The distribution of Jurassic ages suggests that prior to the emplacement of the Cretaceous granitoids, Jurassic granitoids were widely distributed across the central Sierra Nevada but were not emplaced in a west-to-east succession as were the Cretaceous granitoids. Few of our ages fall between 155 and 125 m.y. However, a U-Pb age of 144 m.y. has been reported on the Sage Hen Flat pluton in the White Mountains, and U-Pb ages between 134 and 128 m.y. have been reported on remnants of older granitoids farther south in the Sierra Nevada, which are associated with roof pendants and septa. Also, numerous K-Ar ages on hornblende in the range of 152 to 131 m.y. have been reported on samples collected farther north along the west side of the batholith. The distribution of U-Pb ages is consistent with the interpretation that in the central Sierra Nevada, a belt of Cretaceous granitoids trending about N. 20° W. crosses a belt of Jurassic granitoids trending about N. 40° W. However, the U-Pb ages provide little support for the existence of five cyclic intrusive epochs for California and western Nevada. Comparison of the U-Pb ages on zircon with the K-Ar ages on biotite and hornblende shows generally good agreement for the younger granitoids but decreasing agreement for increasingly older granitoids. Most of the K-Ar ages on biotite and many on hornblende from older granitoids appear to have been reduced as a result of reheating by younger plutons. The dispersion of K-Ar ages reflects the complex structural and thermal history of the batholith.

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Five Zirconi samples from Pennsylvania, Virginia, and North Carolina yield discordant uranium-lead ages which suggest an original age of 820 million years and an episodic lead loss at 240 million years. The indicated age of lead loss is interpreted as the age of movement of the Blue Ridge thrust sheet.

1955 fieldwork. Rezak performed the majority of the ship board analyses, and Neuman and Walter took and cared for the cores. Help also was received with core handling from Donald Stewart in 1955 and from James Double in 1956.Fronr their work leading toward publication of a separate report on the Foraminifera, Ruth Todd and Doris Low have provided information on occurrence of the principal foraminiferal species; and ~iiss Low undertook the onerous responsibility of typing, checking, and processing the often con1plicated pages of this report. Kenneth Lohman studied selected samples for diatoms in the midst of heavy pressure from other work. Biological determinations have also been provided by specialists of the U.S. National 11 useum and other organizations as indicated on tables 3, 10, and 11. The fishes were identified by Leonard Schultz, of the U.S. National Museum, and plankton collections were reported on by David L. O'Berry, of the University of Miami Marine Laboratory. As usual, Frederiek M. Bayer has been a constantly available and generous source of information and advice on ecological subjects.Arrangements for work at Andros Town in 1955 were made by Kay Norton and Arne Lindroth of the Wenner-Gren Foundation. At Andros Town much courteous help was received from Gunnar Lindstrs3m, Kenneth Sundin, and Misse Peterson.Both in 1955 and in 1956, F. G. Walton Sn1ith and Hilary Moore came to the rescue at the crucial mon1ent with marine equipment from the University of Mian1i Marine Laboratory. To them and to G. E. Voss, of the Marine Laboratory, and Robert Ginsburg, of Shell Development Corp., I am warmly grateful for rnany favors and much useful advice. To Farrington Daniels I am also grateful for taking tirne out from a very busy period in Washington, D.C., for discussion of the precipitation problem and for subsequent helpful correspondence.An emergency grant of $785 fron1 the Geological Society of America permitted F. D. Sisler's employment for critically needed bacteriological work until arrangements could be completed for his assignrnent to the U.S. Geological Survey. The Department of Geology, U.S. National Museum, generously provided laboratory space and facilities for his work for the duration of this project.Most of the collaborating authors have read and criticized all sections of the report, and D. L. Graf and R. S. Boardn1an critiqued the entire report. Helpful review of individual sections was made by F.

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