6T.J0HN.J MESOZOIC AREAS. 491 



MESOZOIC AREAS. 



From our present uuderstauding of the occurrence of the various 

 geological formations in the district, it is api)arent that over its entire 

 extent prevailed conditions favorable for the accnmnlation of groups of 

 strata representative of one or more of the great periods into -which the 

 oMesozoic era was divided. Hence, in their distribution, the Mcsozoic 

 forjnations are generally found in one or other or both sides of the up- 

 lifted Pakeozoic areas, where they occur in belts of greater or less ex- 

 tent, according to the varying conditions to which they have been sub- 

 jected subsequent to their ui^heaval in the mountain borders. In the 

 loftiest mountain elevations, as in the Teton Itange, denudation has et- 

 fected their almost total removal, limiting their occurrence to zones low 

 in the outlying flanks, where they are in many instances completely hid- 

 den from sight by eruptive and detrital accumulations. But besides 

 these areas, there are also i^erhaps still more extensive belts which may 

 almost be regarded as isolated, forming the bulk of mountain ridges, as 

 instanced in the Caribou Range, where the Mesozoic beds together with 

 still more recent formations have been thrown into a most extraordinary 

 series of flexiu-es and folds. In the following images brief mention is 

 made of the present distribution of the various areas in which rocks 

 of this era occiu', and the more or less local asi)ects under which the 

 component formations appear within the district. 



Toward the south end of the Blackfoot Range, on the southwest 

 flank, a limited exposure of Jurassic is found, which impinges on the Car- 

 boniferous rocks in the mountain ridge in a position such as leads to the 

 conclusion that the strata are faidted, the Jm^assic beds being a remnant 

 of the west-side downthrow. A series of low secondary ridges south 

 of the Blackfoot Range, at Stations XII and XIII, are made up of well- 

 defined Jurassic beds which have been upraised into a symmetrical anti- 

 clinal, beyond which occurs a much more complicated belt occupying the 

 space intervening between these ridges and a higher outlying Carbonifer- 

 ous ridge on the north. The same series of strata reappear on the south- 

 west side of the Blackfoot on the extreme southern border of the dis- 

 trict, but soon disappear beneath the Pliocene and volcanic deposits, 

 which latter occupy a broad belt on the eastern flank of the low ridge 

 extending south from Higliam's Peak. In the latter divide the Mesozoics, 

 Jurassic, and jiossibly Triassic, form the axis of a synclinal, which to 

 the soutli also nearly corresponds to the water-divide of the Ross Fork- 

 Portneuf and Blackfoot drainage. The end of the ridge on the west 

 side of Lincoln Creek, according to Professor Bradley, shows a heavy 

 jilating of the same series. On the eastern side cf this synclinal, the 

 strata were involved in tremendous disturbance, the Carboniferous stand- 

 ing in ^'ertical ledges, and the Jurassic also showing very irreg'ular and 

 A'ariable dipa, as though two distinct sets of disturbing forces had here 

 met, throwing the strata into greatest confusion. 



In the before-mentioned areas, where these rocks are in intimate asso- 

 ciation with Palneoz'oic areas, interesting examples are met with in the 

 low ridges west of John Gray's Lake, sliowing a heavy series of strata, 

 probably including both Triassic and Jurassic deposits, which seem to 

 partially fold round their extremities, the nucleal rocks consisting of 

 Carboniferous deposits. In the Caribou Range the same deposits ai)pear 

 in two or three anticlinal folds, in places inverted and possibly faulted, 

 the direction of the forces concerned in the disturbance very nearly cor- 

 responding with that of the present mountain range. The middle por- 



