EOCENE IGNEOUS ROCKS 105 



graphic break since Algonkian time. The oldest intrusives, recognized 

 as such, are found in the northwestern corner of the Yellowstone Park 

 in what is now the Gallatin Eange, and inaugurated a physical revolu- 

 tion. These rocks were forced in as sills between Upper Mesozoic sand- 

 stones before the latter were much disturbed, as they lie unconformably 

 interbedded between sediments which later were affected by the dynamic 

 processes of mountain elevation. In this sense these earliest intrusions 

 must be considered, structurally at least, as of Cretaceous age. They 

 were succeeded by more powerful injections, accompanied by slow and 

 protracted elevation of the Gallatin Eange. With the emergence of land 

 surfaces erosion followed and sediments were deposited unconformably. 

 Elevation of mountain masses produced new physiographic features, and 

 as a consequence changes in climatic conditions and modifications of liv- 

 ing species, both animal and vegetable. The Tertiary period was ushered 

 in. With the progressive building up of the range and the associated 

 folding and compression of strata, viscous magmas were injected from 

 unknown depths. Massive bodies were forced upward to definite levels, 

 when, being unable to rise higher, they spread out laterally between 

 strata of all ages, from the Cambrian to the Laramie. Centers of power- 

 ful intrusion shifted from one locality to another, and within the con- 

 fined limits of the range batholiths of no mean proportions were forced 

 upward. Evidence is wanting to show that any of these magmas in their 

 upward movement ever penetrated to the surface; apparently they came 

 to a standstill far below a covering of overlying sediments, whose thick- 

 ness must, for the present at least, remain a matter of conjecture. Exces- 

 sive erosion since early Eocene time has laid bare these massive batho- 

 lithic forms, which now stand out as dominant features in the landscape. 



There is no evidence to warrant the opinion that these porphyries and 

 crystalline rocks were ever connected with vents discharging lavas, tliough 

 there is, beyond the boundaries of the Gallatin Range, extrava sated ma- 

 terial of Eocene age covering large tracts of country. In the nortlieast 

 corner of the park such surface flows are well developed in the accumula- 

 tion of silts and ashes. Much of this material was laid down under 

 relatively quiet conditions. Apparently they are much later than the 

 crystalline rocks already referred to, biit their age is determined by a 

 characteristic flora corresponding with the well known Fort Union beds 

 of Montana of Eocene age. 



In these extravasated lavas the influence of volcanic waters may be 

 recognized in many ways, but degradation of the mass has been so great 

 that evidence of individual extinct hot springs is no longer traceable; 

 moreover, it would seem impossible to distinguish them from those be- 

 longing to Miocene eruptions. 



