ERUPTIVE ROCKS FROM MONTANA. 43 



plagioclase with a varying' amount of orthoclase) and 

 sometimes quartz. There may be two generations of 

 quartz and feldspar but only one early generation of 

 hornblende. These rocks, although varying much in 

 structure and composition, apparently form a natural 

 group and occur in the Little Belt Mountains as large in- 

 trusive masses — evidently laccolites — in Carboniferous 

 and Jurassic strata ; along Dearborn Creek on the east 

 side of the Main Range on the trail to Cadottes* Pass, 

 rocks indistinguishable from the former occur as dikes in 

 Cretaceous or Laramie sandstones. The eruptives from 

 both these localities are indeed so similar that they may 

 be described together. 



The most prevalent habit is porph\ritic, but there also 

 appears to be a continuous series of transition from por- 

 phvritic to tine granular hvphidiomorphic rocks. The 

 color of the rocks is usually yellowish or yellowish gray : 

 in a groundmass of fine-grained structure are imbedded 

 phenocrysts of a glass}^ fresh feldspar, smaller, well de- 

 lined, usually rectangular feldspar crystals of a yellowish 

 color and small quartz grains, sparingl}' distributed; bio- 

 tite foils occur in some specimens, but universally present 

 are prisms of green hornblende, not more than 2—3 mm. 

 in length. 



Under the microscope the rocks of this class present the 

 following characteristics: Larger, not striated feldspar 

 phenocrysts of sometimes irregular outlines : this is, accord- 

 ing to extinctions obtained from cleavage fragments, or- 

 thoclase, and is present in varying quantities. There is 

 indeed reason to believe that these rocks by gradual tran- 

 sition go over into trachytic and rhyolitic forms. Usually 

 much more abundant are square or rectangular sections 

 of a triclinic soda-lime-feldspar which, according to the 

 extinctions of the twin lamelhe should be referred to an- 



