MADISON RANGE. 59 



ward down to the valley of the Gallatin River, the dip being about 20°. 

 The summit of the mountain is flat, and shows Carboniferous limestones 

 dipping northeast at gentle angles. This mountain and the one north of 

 it both show the characteristic flat-topped topography noticed in the lesser 

 marginal peaks of the Madison Range. 



The andesite-porphyry of the laccolith is quite like those rocks in the 

 Gallatin already described, and its petrographic description is given in 

 another chapter. A characteristic feature of the exposures seen of this rock 

 is the occurrence of numerous included fragments of gneiss, schist, and 

 hornblende-porphyry. The two patches of rhyolite which occur upon the 

 slopes rest directly upon this andesite-porphyry, showing a thorough dis- 

 section of the laccolithic fold before the outpouring of the rhyolite flows. 



