118 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



narrow, slab-like block of strata ranging in age from the Altyn to the Mae- 

 Donald inclusive. It is not easy to understand the conditions under which 

 this narrow slab, composed of the oldest sediments in the range, can 

 make contact on each side with the youngest formation of the range. Two 

 hypotheses are conceivable. According to the first the relations were established 

 by simple normal faulting whereby the Carboniferous blocks were dropped 

 down. According to the second hypothesis one may postulate a local overthrust 

 of the older rocks upon the Carboniferous, followed by normal faulting which 

 dropped the Altyn-MacDonald block down into the Carboniferous limestone. 

 This second view would naturally correlate with the speculation that the whole 

 Clarke range has been thrust over Carboniferous or Cretaceous formations. 

 Extreme as this idea may be, the known facts do not exclude it and the two 

 hypothetical alternatives are still open. It may be noted that the outcrops of 

 the fault planes on the east side of the Altyn-MacDonald slab have been drawn 

 with considerable confidence. The fault line on the western side was not so 

 readily plotted in the field, but it is believed to be mapped with approximate 

 accuracy. 



In four or more leading cases the faul , lines have been shown as following 

 stream courses among these mountains. The local valley of Wigwam river lias 

 been determined in position by a break which has strongly affected the dips of 

 the MaeDonald formation on either side of the river. 



The five most westerly blocks of the Galton range show a progressive clown- 

 dropping of blocks from east to west, wit i the remit that the MaeDonald. 

 Siyeh, Purcell, Gateway, and Devonian-limestone formations are successively 

 in lateral contact. The equivalence of level between the Devonian limestone 

 at Tobacco Plains and the lower MaeDonald beds along the Wig-warn show- 

 that the net relative displacement of the two blocks has been at least 10,000 

 feet and may have been several thousand feet greater. We are therefore pre- 

 pared to find that the Rocky Mountain Trench at the Forty-ninth Parallel has 

 been located on a zone of strong faulting. This conclusion will be noted again, 

 in the next chapter, on the Purcell mountain system. 



The relation of the Devonian and Carboniferous formations to the older 

 geosynclinal prism has been discussed in connection with the stratigraphy of 

 the younger limestone-. The Boundary belt has furnished very little .informa- 

 tion on this subject. 



