ERUrXIVK ROCKS FROAI .MONTANA. 4I 



It is not easy or even possible in a great many cases to 

 establish the exact age of these eruptives ; this is largely 

 owing to the fact that no rocks of later age than Laramie, 

 the disputed territory between the Cretaceous and the 

 Tertiary, occur in the region referred to. The volcanics 

 are intrusive in sedimentary rocks of very different age, 

 from Cambrian to Laramie ; nearly all of the types may, 

 however, be found in Cretaceous or Laramie strata, and 

 the evidence tends to show that the eruptions, beginning 

 towards the close of the Cretaceous period, continued 

 during part of the Tertiary. During the later part of the 

 Tertiary and Quaternary the eruptions must have sub- 

 sided in Northern and Central Montana, although further 

 southward they still continued with undiminished force. 



The character of the subaerial masses accompanying 

 these eruptions is not well known ; only a few conglom- 

 erates in the Laramie give some hints as to their nature. 

 In the case of a volcanic conglomerate at the coal fields 

 of Bozeman the horizon could be determined to be 2200' 

 abo^•e strata in which fossils of the Fort Benton group 

 were found ; this conglomerate consists of pebbles of 

 hornblende-andesites to which consequently no later age 

 than Lower Laramie can be assigned. 



In a conglomerate in the Ilighwood Mts. (Laramie or 

 Upper Cretaceous) dacites and andesites with brown, 

 black-bordered hornblende and cryptocrvstalline ground- 

 mass are noticed. At Sixteen-Mile Creek (Belt Mts.) 

 augiteandesite with glassy microlitic groundmass is found 

 in a conglomerate, interbedded with Laramie strata.* 



The nomenclature of this Cretaceo-tertiary series of in- 

 trusives offers a great many difficulties. I have in this 

 paper used the names of the tertiary effusive rocks for 

 the different porphyritic members of the series, and, for 



^loth Census, vol. w, p. 736, 



