50 GEOLOGY OF THE DENVEK BASIN. 



3. Transverse folds. Next tO be considered are the (WCllCS of till' srit -linttotU 



ill Golden and Boulder, whose axes, as has already been stated, must have 

 been more <>r less at right angles with the prevailing line of strike. These 

 arches were formed previous t<> the main or post-Laramie movement, and 

 must be conceived to be the result of a longitudinal ('(impression. The 

 writer's study of mountain uplift has shown him that in most every great 

 mountain uplift there are evidences of two forces of compression acting 

 more or less at right angles to each other, as here, and producing a major 

 and a minor scries of folds. In the Archean area immediately adjoining 

 the present held, indeed, the structural lines show evidence of considerable 

 longitudinal compression. Hence the force which produced these arches 

 is not difficult to conceive of, hut it is less easy to understand why the arch 

 should have occurred exactly where it did and not at other points. Once 

 initiated, it is quite comprehensible that it should ever afterward be a 

 point of weakness, or of least resistance to the compressing forces, and 

 thus the succeeding movements are readily conceivable. That it continued 

 to be a line of weakness after longitudinal compression had ceased is also 

 evidenced by the fact that it was at this locality in Denver time that the 

 basaltic eruptions were forced up to the surface. 



It might be argued that the extravasation of such a considerable 

 mass of eruptive material from beneath the surface would account, in part 

 at least, for the sinking of the upper part of the sedimentary series at this 

 point (which is predicated by Mr. Eldridge's diagram, p. 99, in order to 

 account for the present relative position of their outcrops), and that this 

 would obviate the difficulty conceivable in the ironing out or flattening of 

 the arch previous to the upturning of the beds along the flanks of the range. 

 This would, however, necessitate the assumption that the greater part of 

 this upturning occurred since the deposition of the Denver beds. If it were 

 also assumed, as has been already suggested as possible, that the upper 

 part of the sedimentary series above the Middle Cretaceous clays had been 

 pushed in toward the foothills, prior to this upturning, by an overthrust 

 movement over the arch, there would have been less of an arch to be 

 planed down and the void left by the extravasation of the basalt might 

 have been quite sufficient to have produced the flattening of the arch. 



