This paper concerns the Southern Rocky Mountains, lying chiefly in Colorado. Certain recorded observations relative to them are recalled and possible causes of their rise discussed. It is shown that the theory of lateral compression, frequently advanced as the cause of mountain-building and recently applied by R. T. Chamberlin to the Colorado Rockies,2 is not in harmony with many of the observed facts, and the question is raised, if changes in temperature, density, mineral constitution, etcetera, can explain such elevations and depressions as are known to have occurred. The building of the Southern Rockies might be discussed from the standpoint of probability and possibility, but when requested to speak on this subject it seemed to me that the only way in which I could contribute to the much-discussed question of mountain-building would be to outline a definite problem and to seek for such quantitative data as might help in . . .