st. john-.J MESOZOIC AREAS. 491 



MESOZOIC AEEAS. 



From our present understanding of the occurrence of the various 

 geological formations hi the district, it is apparent that over its entire 

 extent prevailed conditions favorable for the accumulation of groups of 

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

 Mesozoic era was divided. Hence, in their distribution, the Mesozoic 

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

 lifted Paleozoic 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 upheaval in the mountain borders. In the 

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

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

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

 den from sight by eruptive and detrital accumulations. But besides 

 these areas, there are also perhaps still more extensive belts which may 

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

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

 still more recent formations have been thrown into a most extraordinary 

 series of flexures and folds. In the following pages brief mention is 

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

 of this era occur, and the more or less local aspects under which the 

 component formations appear within the district. 



Toward the south end of the Blackfoot Eange, 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 Jurassic beds being a remnant 

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

 of the Blackfoot Eange, 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- 

 ons 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 Higham's Peak. In the latter divide the Mesozoics, 

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

 the south also nearly corresponds to the water-divide of the Boss Fork- 

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

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

 plating of the same series. On the eastern side of this synclinal, the 

 strata were involved in tremendous disturbance, the Carboniferous stand- 

 ing in vertical ledges, and the Jurassic also showing very irregular and 

 variable dips, 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 Pakeozoic areas, interesting examples are met with in the 

 low ridges west of John Gray's Lake, showing 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 Eange the same deposits appear 

 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- 



