In the arid Western regions these formations girdle the Rockies and outlying mountain chains for two-thousand miles from north to south, and are extensively exposed in great escarpments, river canyons and "badland" areas, bare of soil and vegetation and affording an immense stretch of exposed rock for the explorer.

Much of this area indeed is desert, too far away from water to be profitably searched under present conditions, or too far away from railroads to allow of transportation of the finds at a reasonable expense.

Fossils are much more common in certain parts of the region, and these localities have mostly been explored more or less thoroughly.

But the field is far from being exhausted.

New localities have been found and old localities re-explored in recent years, yielding specimens equal to or better than any heretofore discovered.