1914] Buwalda: Tertiary Mammal Beds in West-Central Nevada 353 



fanglomerate mantle commonly does not extend up the slopes of the 

 adjacent ranges, probably because these slopes are usually too steep 

 for the lodgment of the terrestrial materials. The mantle consists 

 of coarse, angular fragments of the various rocks of the surrounding 

 ranges, such as limestone, marble, plutonic rocks, and lavas, all em- 

 bedded in an arkosic matrix derived from the same source. Tbe 

 mantle is strictly similar in every respect to the deposits now flooring 

 the wide washes which are cutting backward and laterally into the 

 Esmeralda strata. The deformed Esmeralda strata were apparently 

 bevelled off to an even surface and the terrestrial materials laid down 

 unconformably across their edges. The bevelling of the strata by 

 headward erosion and lateral cutting by drainage channels similar to 

 those now active, and the lateral spreading of the alluvial mantle over 

 the cut surface probably went on concomitantly ; reasoning from 

 present to past, the mantle was spread progressively by the water- 

 courses which did the cutting over the surface produced on the strata 

 as rapidly as it was cut, instead of the surface being first entirely 

 formed over a large area of the beds and then subsequently covered 

 by the mantle through some change of condition. The materials form- 

 ing the mantle were derived in part from the mantles on the older 

 mesa surfaces above, which were being destroyed, and in part were 

 probably brought from the mountains by the washes over the upper 

 mesa surfaces. 



No organic remains were obtained from the fanglomerate mantles. 

 Since the different mesas were not cut contemporaneously, the lowest 

 set having been formed last, it is obvious that the fanglomerates cap- 

 ping the different areas are of somewhat different ages. The mantles 

 are considerably indurated and quite resistant, but apparently all lie 

 in the attitude in which they were deposited, sloping one to two 

 degrees from the mountain flanks toward the middle of the valley. 

 Their surfaces commonly are so-called desert pavements, formed by 

 the removal, through wind and water, of the finer materials in the 

 top layer of the fanglomerate and the smoothing down of the remain- 

 ing coarse fragments into a flat pavement. It is probable that the 

 oldest of the fanglomerate mantles was formed before the latest 

 Pleistocene. 



Southern G-abbs Valcey Range 

 There are areas of old metamorphic limestone in the southern 

 Gabbs Valley Range lithologically similar to the limestone in Cedar 



