42 CALIFORNIA ACADEMY OF SCIENCES. 



the granular rocks of the same age, the names of diorite, 

 syenite, etc. The latter have been employed simply as 

 structural and mineralogical terms, and not as implying 

 any certain age. 



Among the dacites and hornblende andesites are found 

 many which, by other writers, doubtless would have been 

 classed as porphyrites on account of their intrusive occur- 

 rence and holocrystalline structure. 



A brief reference should be made to the few igneous 

 rocks in the district examined, which are not with any cer- 

 tainty connected with this later Cretaceo-tertiarv series. 

 Well exposed at Mullan Pass and between there and 

 Helena is an area of hornblende biotite granite which at 

 its contact metamorphoses the adjoining carboniferous 

 limestones; it is rich in plagioclase and may in places 

 rather be considered as a quartz mica diorite. The series 

 exposed at Mullan Pass extends from the Carboniferous 

 to the Cretaceous, and the possibility is not excluded, in- 

 deed, that the granite is of very late Mesozoic age. 



Similar intrusive masses appear in the Big Belt Mount- 

 ains and, connected with them, dikes of quartz-porph^-rite 

 in the Cambrian slates. 



Dikes of diabase have been observed in the red Cam- 

 brian or Silurian slates at several places and only in these 

 strata. It is partly a normal diabase, partly a quartz dia- 

 base, the quartz being connected with the feldspar in 

 granophyric structure.* 



I. Dacites, Hornblende-Anuesites, Diorites. 



In the Little Belt Mountains and at various points in 

 front of the Main Range, west of Fort Benton, are found 

 light-colored, mostly porphyritic, more or less acid rocks, 

 principally composed of hornblende, feldspar (usually 



*loth Census, vol. xv, p. 735. 



