THE FAN. 45 



above the Dakota conglomerate the shales are split by two thin sheets of 

 andesite-porphyry, and also by a massive layer that appears as a small lac- 

 colith constituting the northwestern end of the ridge. The petrographical 

 character of the sheets is not the same in all cases, but the distinctions are 

 slight and will be discussed in Chapter II. In the high ridge between the 

 head branches of Gardiner River, at least five different sheets of andesite- 

 porphyry were observed, having the general dip of the shales and sandstones, 

 with occasional ruptures across the beds. Tlie}^ were also found crossing 

 the valley to the north and forming part of the ridge leading to Electric 

 Peak, as represented on the map. Their thicknesses are not constant, as may 

 be seen in their exposures, but the actual variation is greatly exaggerated 

 in appearance by the positions of the exposures, whether directly across the 

 thickness or more or less parallel to the sheet. This impression is still 

 further increased by the appearance produced by the more persistent talus 

 slopes of the harder porphyry, which often obscure more easily removable 

 areas of the softer shale. The same sheets occur in the shales in the upper 

 part of Little Quadrant Mountain. 



THE FAN. 



Fan Creek drains the mountainous area whose various spurs converge 

 to the west at the ribs of the Fan, from which resemblance the region 

 derives its name. The encircling ridge which forms the divide between 

 the waters of Fan Creek and those of the Gallatin and Gardiner drainages 

 culminates in two prominent peaks, one of which, Gray Peak, has already 

 been described. The other, lying to the north, is named Joseph Peak, and 

 occupies a commanding position just west of Little Quadrant Mountain. 

 The southern fork of Fan Creek is named Stellaria Creek. At the head of 

 this stream the high ridge which is the southwestern extension of Gray 

 Peak is formed of Mesozoic beds, having a strike of N. 20° E. and a dip of 

 10° W. The peak is formed of Dakota sandstone resting upon the Jura- 

 trias beds, and is cut by intrusive sheets of andesite-porphyry from the 

 Gray Peak bysmalith, as already described on page 42. 



