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92 At most places, the lower part of the Pyrotherium beds contain hard tuffs, weathering brown, + vesicular, although these are occasionally lacking. In profile AA the Pyrotherium jaw (although its horizon is not fixed within 15') defines the base of the beds at or near a similar hard brown bed and there are numerous bones + frag. teeth in the base of the formation. In AB no fossils were found but the presence of an unconformity and of beds similar lithologically places the equivalence as indicated, beyond much doubt. In all three sections a pale buff to pink tuff with hard silicified patches or concretions occurs about 100'-125' above the base of the Pyrotherium beds, without any apparent intervening break. It is highly probable that this distinctive bed is the same in the three cases. In AA it contains rare Strophocheilus and a few identifiable mammals. In AB it was found barren. In AC it contains abundant Strophocheilus and some bone fragments, but no identifiable mammals were found. In section AC a still higher series is exposed, as shown, but no identifiable fossils were found. 93 AGE OF FOLDING: The presence of a fold here with dips relatively high for the Tertiary (up to 30�) seems very important. Even though somewhat obscured by slumping and small faults, this fold (monoclinal in the part observed but perhaps one limb of an anticline the other limb of which is buried in the valley) seems to be antecedent to the present topography, unlike the supposed fields in the arg. fis. south of Coli-Huapi. With almost constant strike and no unusual or sudden changes in dip it follows for some distance. Its axis is roughly parallel to that o the anticlines of Cerro Chenque, Cerro Castillo, etc., and it lies almost in line with the former. It seems possible that it is part of the same system of folds, often supposed to be pre- or eo-Tertiary. Here, at least, there appears to have been no marked folding before the Pyrotheriano. There is no evidence of any folding to the top of the Notostylopense. The beds are sensibly parallel + folding, if present, was too slight to be observed in small sections. There is some, but uncertain, evidence