Page:The National geographic magazine, volume 1.djvu/36

 it is worn down to a featureless base level surface, are worthy of the most attentive study. The immaturity of the broken country of southern Oregon, as compared with the more advanced forms of the Basin ranges, is a case in hand. The Triassic formation of the Connecticut valley is in some ways of similar structure, being broken by long parallel faults into narrow blocks or slabs, every block being tilted from its original position. Russell's description of the blocks in southern Oregon would apply nicely to those in Connecticut, except that the former have diverse displacements, while the latter all dip one way; but the Connecticut individual has, I feel confident, passed through one cycle of life and has entered well on a second; it has once been worn down nearly to base level since it was broken and faulted, and subsequent elevation at a rather remote period has allowed good advance in a repetition of this process. The general uniformity in the height of its trap ridges and their strong relief above the present broad valley bottom, require us to suppose this complexity of history. A given structure may therefore pass through two or more successive cycles of life, and before considering the resulting composite history in its entirety, it would be best to examine cases of simple development in a single cycle. After this is accomplished, it would be possible to recognize the incomplete partial cycles through which a structure has passed, and to refer every detail of form to the cycle in which it was produced.

The most elementary example that may be chosen to illustrate a simple cycle of geographic life is that of a plain, elevated to a moderate height above its base level. The case has already been referred to here and is given in more detail in an article printed in the proceedings of the American Association for the Advancement of Science, for 1884, to which I would now refer. When the succession of forms there described as developed at a given elevation over base level is clearly perceived, the occurrence of forms dependent on two different base levels in a single region can easily be recognized. The most striking example of such a complex case that I know of is that of the high plateaus of Utah, as described by Dutton. Northern New Jersey presents another example less striking but no less valuable: the general upland surface of the Highlands is an old base level, in which valleys have been cut in consequence of a subsequent elevation. The plateau developed on the tilted Triassic beds about Bound Brook is a second base level, cut during a halt in the rise from the