AGE OF UPLIFT. 81 



of these beds made during the present investigation furnishes some evidence 

 of a shallowing of these seas, and perhaps even of the existence of some 

 land surfaces subjected to erosion during part of this time. Still, the absence 

 of non-conformity in the successive strata deposited and their great uni- 

 formity throughout the area studied show that no violent dynamic move- 

 ment took place before the great disturbance at the close of the Cretaceous, 

 which extended throughout the whole of the Rocky Mountain S3'stem and 

 was doubtless the main factor in [)roducing its present elevation. 



During this long period of conformable deposition there was an accu- 

 mulation in this area of 10,000 to 12,000 feet of sedimentary beds. Toward 

 the latter part of this period, possibly very near its close, there was an exhi- 

 bition of intense eruptive activity, during which enormous masses of molten 

 rock were intruded tlu-ougli the underlying Archean Hoor into the overly- 

 ing sedimentary deposits, crossing the beds to greater or less elevations 

 and then spreading out in immense sheets along the planes of division 

 between the different strata. It is not possible at present to define all the 

 points at which these eruptive masses forced their way up, although ihej 

 were doubtless very numerous and widely spread throughout the region; 

 but tlie negative evidence obtained proves that the intrusive force must 

 have been almost inconceivabl}' g'l'^'at, since comparatively thin sheets of 

 molten rock were forced continuously for distances of many miles between 

 the sedimentary beds. That the eruptions were intermittent and continued 

 during a considerable lapse of time is })roved by the great variety of erup- 

 tive rocks now found and by the fact tliat a given rock in one place pre- 

 cedes and in another follows a second. It might naturtilly be thought that 

 this eruptive activity must have been coincident with or immediately sub- 

 sequent to a great d3-namic movement; but that it preceded the movement 

 at the close of the Cretaceous, which caused the uplift of the Mosquito 

 Range as well as of the other Rocky Mountain Ranges, is proved by the 

 fact that these interbedded sheets of eruptive rocks, porphyries and porphy- 

 rites, are found practically conformable with their bounding strata, and, like 

 them, folded into sharp folds and cut oft' by faults. The intrusion between 

 the strata of such vast masses of rock — which in some cases reached a 

 thickness of from 1,000 feet to 2,000 i'eet, and of Avhich in other cases sue- 



