612 PROF. J. P. IDDINGS ON EXTRUSIVE [Nov. 1 896, 



forms a laccolite in Cambrian strata immediately over gneiss. Its 

 volume was about 1| cubic mile, and its silica content is 61 per cent. 

 It is traversed by a second plug-like intrusion, whose volume was 

 probably 2 cubic miles : the average silica content of this mass is 

 about 70 per cent. A third body of intrusive rock is like the first in 

 composition, having about 65 per cent, of silica. These are the oldest 

 intrusions exposed in the Gallatin Mountains. The intrusive sheets 

 connected with the last-mentioned bodies traverse Electric Peak 

 and are older than the eruption of igneous rocks that rose through 

 the conduit there and reached the surface as andesitic breccias, and 

 filled dykes and the stock with porphyries and diorite. At this 

 place the intrusive and extrusive series are directly connected, and 

 the two groups correspond in the order of their eruption. The 

 rocks flowing through the conduit appeared on the surface as horn- 

 blende-pyroxene-andesites cut by a dyke of basaltic andesite, and 

 by more siliceous andesite-porphyries, ending in dacite: the in- 

 trusive series ranging from diorite to quartz-mica-diorite and 

 granite. The volumes of these bodies are small, the cross-section 

 of the core being about J square mile. 



In the neighbourhood of Haystack Mountain, at the head of 

 Boulder Creek, Montana, the sheets intruded into the Cambrian beds 

 are acid andesitic porphyries, which appear to have been connected 

 with the earliest eruptions of acid andesitic breccias. In the core 

 of the Haystack volcano the intruded mass is gabbro-diorite, 

 varying in different parts between these two varieties of rocks. It 

 is cut by a few small dykes of more siliceous rock. The region 

 about Emigrant Peak contains several large bodies of dacite and 

 dacite-porphyry that burst up through the basic andesitic breccia. 

 The superficial extent of these intrusive bodies approaches 40 square 

 miles, and their volume was probably 20 cubic miles. 



The core of the Crandall volcano consists of gabbro, passing into 

 diorite, with a constantly increasing acidity, the last intrusions 

 being granite. 1 The cross -section of this core is about 1 \ square 

 mile, and, if 10,000 feet deep, its volume was about 3 cubic miles. 

 The most siliceous portions are relatively very small. The dykes 

 which radiate from this core are mostly formed of rocks correspond- 

 ing to gabbro in chemical composition, only a few correspond to 

 the diorite, and none were found which were equivalent to the 

 granite. Some, however, occur more basic than the gabbro, and 

 there are complementary dykes of absarokite and banakite. 2 In the 

 breccia in places are the lava-flows corresponding to these comple- 

 mentary dykes, but their volume is quite insignificant. There are 

 similar occurrences of dykes and superficial lava-streams throughout 

 the range of mountains to the south. 



The eruptions of these intrusive rocks were contemporaneous 

 with those of the andesitic breccias, in many instances, and occurred 

 within the period during which the volcanic range was built. They 

 have suffered erosion together with this range. The coarse-grained 



1 Op. cit. p. 609. 



2 Iddings, ' Absarokite-Shoshonite-Banakite Series,' Journal of Geology, 

 vol. iii. (1895) no. 8. 



