346 REPORT UNITED STATES GEOLOGICAL SURVEY. 



clinal, recourse must be had to something more than mere folding of the 

 strata, a force which severed their stratigraphic continuity by a fault, 

 with a downthrow on the southwest amounting probably to at least two 

 or three thousand feet. The strata in the adjacent mountain-crest may 

 be safely regarded as holding a position below the middle of the Car- 

 boniferous series, as developed in this region, and the downthrow lias 

 involved the remaining upper portion of the Carboniferous, tlie whole 

 of the Trias, and the lower measures of the Jurassic. Tracing the course 

 of the fault, it apparently soon runs out into the Blackfoot Valley to the 

 northwest, while in the opposite direction it skirts the southwest flank 

 of the Blackfoot Kange and disappears beneath the volcanics which en- 

 velope the southern terminus of the range. 



Passing around the southern end of the Blackfoot Range, our way led 

 up over a highish platform of basalt, which appeared in rugged, sombre 

 expos ures, and at one point presented the appearance of a cascade, the 

 igneous matter, as it were, having overflowed a depression from the east- 

 ern basin, forming a connection with the flow that filled the Blackfoot 

 Valley. This locality may be in the neighborhood of three miles south- 

 eastward of Station XL Immediately to the south, in the summit of 

 the low divide in which the range is continued a little farther, the bluish- 

 gray spar-seamed Carboniferous limestones show in limited exposures, 

 dipping off to the northeast ; and still farther to the southward, in the 

 extreme end of the divide facing the upper basin of the Blackfoot, at 

 a later date of our visit similar deposits were observed to form a syn- 

 clinal, the axis of which lies a little to the southwest of the crest of the 

 divide, which here rises in a gentle swell but little elevated above the 

 higher benches of basalt which lap up on and surround the sediment- 

 aries. 



From such data as we were able to obtain, it is apparent that this 

 little mountain range possesses a geological history not so simple as 

 might at first appear. Examined at one point, its structure might be 

 interpreted as a simple monoclinal ridge, resulting from the degradation 

 and complete removal of the opposed side of an anticlinal fold ; again, it 

 shows a similar structure, but which is clearly due to faulting and a con- 

 sequent downthrow of many hundred feet ; and elsewhere its principal 

 ridge conforms to the axis of greatest elevation at one point, and at 

 another the reverse is true, where the crest occupies the axis of a syncli- 

 nal. And with all this diversity, which might be readily deciphered in 

 detail otherwise, there are associated entirely different geological pro- 

 ducts, which, at a date so modern as to have been introduced at a time 

 when the range had received nearly its present configuration, half buried 

 its flanks and crept into and filled its valleys with flows of molten rock 

 from volcanic sources to such an extent that their remnants as found to- 

 day greatly embarrass the study of the dynamic history of the range, 

 not only by hiding beneath their rigid surface the buried sedimentaries, 

 but completely isolating this narrow zone from contiguous and similar 

 areas of disturbance, the inception of which long antedates the manifesta- 

 tion of this widely extended and comparatively modern epoch of vol- 

 canic activity. 



There remains to be mentioned a small area of isolated hills lying be- 

 tween the Blackfoot River and the Blackfoot Range, which is appar- 

 ently quite cut off from connection with neighboring highland areas by 

 the basaltic flow which fills the Blackfoot Valley. Without showing a 

 distinct topographic crest or ridge, yet this area has a well-defined drain- 

 age axis, which was marked out by the nature and position of its rocky 

 foundation. The strata were found to strike pretty uniformly about E. 



