Page:Popular Science Monthly Volume 86.djvu/267

Rh At the present time we are not in a position to state definitely the exact position or relationship of the Manix fauna with relation to other Pleistocene life in the west. The problem of the Pleistocene in this region is complicated and large, and the many elements still require much study before their interrelations can be determined. The Rancho La Brea fauna seems to contain elements similar to those of Manix, whether it is older or younger is not yet entirely clear.

The significance of the Manix fauna in relation to its environment is unfortunately not large. The presence of camels, horses, and antelopes indicates a climate somewhat more humid than that in this region at the present time, and such was the suggestion furnished by Mr. Buwalda's work on the physical history of the Manix Lake basin.

The physical history of the Mohave area, in the time that has passed since the accumulation of the oldest formation containing a mammalian fauna in this basin, is only a small part of the long and complicated geologic story of the region; but the changes that have occurred since the earliest of these records of life were completed take on stupendous proportions when measured against human standards of stability. Since the deposition of the oldest beds of the Barstow section, not less than 8,000 ft. of known sediments have been laid down in this region, and there are evidences of long periods from which the only record that we have is of erosion instead of deposition. The strata of both the Barstow and Ricardo sections have been subjected to extreme movements of the earth's crust in folding and faulting or breaking. They have also been extensively eroded or worn down, and the strata now exposed can be considered only as remnants of the original mass. In terms of accumulation and erosion of deposits, judged by the best estimates that we can make, the lapse of time since burial of the oldest mammal remains in this region must be very long.

Physical changes of great significance in the history of this region, and of the life in it, are also noted in variations in the nature of the bordering mountain ranges. At present the Mohave owes its distinctive characters in large measure to separation from the Pacific coast by high ridges to the west. Throughout a large part of the known life history of this region, a barrier seems to have existed between the Mohave area and the Pacific coast province. The height of the separating wall has presumably varied much, being relatively small in Miocene time, and probably reaching its maximum since the Ricardo Pliocene. Variation in height of the barrier depended on the balance between erosion constantly wearing it down, and on the magnitude of crustal movements concerned in the making of the mountain chains. To some