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Show COMPARISON OF OROGRAPHIC FORMS. 49 or less flexed, but the flexing, according to Mr. Marvine, is chiefly of very ancient dateâ€"certainly Pre-Tertiaiy. Thus the lifting of these platforms has no significance corresponding to an anticlinal fold. It is expressed by the conception of a block of strata having a fault or equivalent monoclinal flexure upon both sides. But while these characteristics predominate strongly throughout the more easterly ranges of the Rocky system numberless changes are rung upon them. One dislocation is usually greater than the other. One fades out to a mere inclined plane, while the other becomes a gigantic fault; all shades of difference are found from the evanishment of one to the sensible equality of both The relative courses of the two displacements constantly vary; here parallel, there converging, and again diverging. But throughout this diversity the dominant type-form is still persistent. These broad platforms have upon their surfaces in most cases a certain amount of minor flexing and undulation. Occasionally a sharp turn of the strata upwards or downwards produces a minor or superimposed wave with a well marked anticlinal and synclinal profile. Minor faults and local shattering are also seen here and there. But those systematic repetitive parallel waves of strata, which are conveyed to the mind when we speak of plication are not found in any known region east of the Sierra Nevada and west of the Apalachians. In the Uintas we find a repetition of the Park Mountain type upon a grand scale. This has been illustrated admirably by Professor Powell in his work on the geology of the Uinta Mountains. It consists of a block somewhat broader than those of Colorado, but otherwise the type presents no essential modification. It has a great monoclinal upon the southern flank and a colossal fault upon the northern. Between the dislocations there is a notable amount of superimposed undulation and subordinate fracturing and flexing; but the greater part of it antedates the Tertiary history of the range, and very much of it is at least as old as the Carboniferous. In the Plateau Province there are very few mountains, and such as occur are of volcanic origin. Some of them are constructed in a most singular manner, presenting in their genesis and structure an utter contrast to the Alpine and most of the Colorado forms. Lenticular masses of igneous 4 H P |
