COMPARISON OP OROGRAPHIC PORMS. 49 



or less flexed, but the flexing, according to Mr. Marvine, is chiefly of very 

 ancient date — certainly Pre-Tertiary. 



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 in- 

 clined plane, while the other becomes a gigantic fault ; all shades of differ- 

 ence 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 diver- 

 sity the dominant type-form is still persistent. These broad platforms have 

 upon their surfaces in most cases a certain amount of minor flexing and un- 

 dulation. 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 Carbon- 

 iferous. 



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 r 



