8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I23 



less than 3 centimeters thick. Some of these beds have an upper 

 layer of clay a few millimeters thick, and some of these clay layers 

 were broken into curved fragments or flakes by drying before the 

 next layer of silt was deposited. This indicates that the area con- 

 sisted of a mud flat that was subject to intermittent flooding and 

 drying at the time the vertebrate remains were buried. The material 

 seems to be bentonitic, for it is firm and hard when dry but swells 

 and then completely disintegrates when placed in water. Each rain- 

 storm therefore washes away a thin surface layer and exposes fresh 

 material for fossil search. Some thin calcite veinlets cut through the 

 beds in different directions, and small calcareous nodules are present 

 though rare. From the fragmentary nature of the fossil remains, it 

 appears that the bones must have been washed into their position of 

 burial. The remains may not have been transported very far, but 

 probably far enough to cause dismemberment of the skeletons and 

 scattering and partial destruction of the bones. No plant remains 

 were noted in any of the nearby beds. 



SIGNIFICANCE OF THE FOSSIL DISCOVERY 



The composition and structure of the lower Cenozoic marine sedi- 

 ments along the coastal plain some 250-300 kilometers east of Guana- 

 juato city indicate, according to Muir (1936, p. 140), that the com- 

 pressive phase of the Laramide orogeny took place there principally 

 in middle Eocene time. He indicates that movements had either de- 

 creased in intensity or nearly ceased by late Eocene time, although 

 they continued on a reduced scale until latest Oligocene time when 

 relative stability was reached. Fries had felt that the earliest Cenozoic 

 reddish nonmarine elastics on the plateau were deposited soon after 

 the culmination of the compressive stage of the Laramide orogeny, 

 but no fossil record for dating these elastics was on hand until the 

 material described in this report was discovered. The late Eocene or 

 early Oligocene age assigned to these fossils from the red conglom- 

 erate near Guanajuato permits for the first time a correlation be- 

 tween the orogenic history as far west as Guanajuato and that along 

 the Gulf coast east of there, indicating that the major compressive 

 orogeny inland had been effected also by late Eocene time. We do 

 not yet know how early the folding took place farther inland, but at 

 least in the eastern part of the State of Queretaro, halfway between 

 Guanajuato and the coastal plain, shales of late Maestrichtian age 

 were affected by the strong folding. We also now know that gentle 

 folding and strong faulting and tilting of fault blocks continued on 



