. Central Asiatic Expeditions of the American Museum of Natural History, under the leadership of Roy Chapman Andrews : preliminary contributions in geology, palaeontology, and zoology. Central Asiatic Expeditions (1921-1930); Scientific expeditions; Natural history; Scientific expeditions; Natural history. 106 Bulletin American Museum of Natural History [Vol. LI NATU'RE OF THE ROCK FLOOR The oldrock floor is a complex of ancient sedimentaiy strata, meta- morphic rocks and intrusive igneous masses, both large and small. The oldest rocks recognized by us are complex injection-gneisses, crj^stall
. Central Asiatic Expeditions of the American Museum of Natural History, under the leadership of Roy Chapman Andrews : preliminary contributions in geology, palaeontology, and zoology. Central Asiatic Expeditions (1921-1930); Scientific expeditions; Natural history; Scientific expeditions; Natural history. 106 Bulletin American Museum of Natural History [Vol. LI NATU'RE OF THE ROCK FLOOR The oldrock floor is a complex of ancient sedimentaiy strata, meta- morphic rocks and intrusive igneous masses, both large and small. The oldest rocks recognized by us are complex injection-gneisses, crj^stalline limestones and related rock types exhibiting the most complex structural conditions and mixtures of composition found in the whole region. They appear to be more deformed, more modified, and richer in injected igneous material than any of the other series, and they are the most confusing to interpret. They should be the oldest of all and may indeed correspond to the Archaean of other lands. On these grounds they are judged to be Archaean, and to represent the T'ai Shan complex described by Willis and Blackwelder as occurring in China proper.^. Ancient crystalline rocks Bathylithlc granite Fig. 4. Cross section from the geologist's field notebook, showing the basin north of Uskuk Mountain. Ancient crystalline rocks—gneisses, schists and interbedded limestones, invaded by granites repre- senting the Great Mongolian Bathylith, form the warped floor on which later sediments have been accumulated. Further deformation with faulting has affected the basin, pushing up the Mt. Uskuk block and making possible development of alluvial fans. Erosion has stripped some of the floor and has carved a broad shallow valley in the sediment, in the almost abandoned depressions of which a salt pan is located. This section is almost continuous with Fig. 10, which lies south of the Uskuk block. The next younger series includes schists and crj^stalline limestones which are found in the Tsetsenwan
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