MOUNT niLGAED. 



271 



Colorado and Basin drainage systems crosses the upper part of Moraine 

 Valley transversely, and the same is true of Summit Valley. 



Mount Hilgard is a lofty headland, rising upon the eastern side of Mo- 

 raine Valley to an altitude of about 11,000 feet. Towards the north and 

 east it presents inaccessible battlements of dark volcanic rock, consisting 

 chiefly of hornblendic trachyte and augitic andesite. Towards the west it 

 presents an abrupt face, which, however, is easily scaled. To the south it 

 extends in a long ridge of diminishing altitude until it reaches the vicinity 

 of Thousand Lake Mountain. To the eastward are seen the sedimentary 

 formations stretching away indefinitely. They are Cretaceous, with a thin 

 fringe of Lower Eocene capping them just at the base of the volcanic wall 

 of which Mount Hilgard forms the loftiest part. To the northward the 

 volcanic wall extends at an altitude 2,000 feet lower than the mountain top, 

 and gradually swings westward until it nearly joins the wall which forms 

 the northern salient at the head of Summit Valley, being divided from it 

 only by a narrow ravine heading near Mount Terrill. This northern 

 extension of the volcanic battlement has been named Gilson's Crest. This, 

 together with the great ridge formed by Mount Hilgard and its southern 

 extension, forms the eastern boundary of the great eruptive masses which 

 cover almost the entire expanse of the District of the High Plateaus. There 

 are, however, two or three outlying patches to the eastward of small extent, 

 evidently independent centers of eruption, but no special significance seemed 

 to attach to them. 



The ridge of which Mount Hilgard is the culmination is evidently a 

 chain of volcanic vents along a fissure, and the extravasation appears to 

 have taken place along its entire extent. There are no individualized 

 peaks or cones suddenly springing up at various points of the chain, but a 

 broad summit platform, slowly and pretty regularly diminishing in altitude 

 through a distance of nearly 20 miles, and it is difficult to point to any par- 

 ticular spot as possessing a more distinctly focal character than the others. 

 The outpoui's appear to have occurred all along the line, with an approxi- 

 mation to uniformity, or possibly with a gradual increase of magnitude and 

 frequency, as we approach the summit of Mount Hilgard. From that head- 

 land southward the top of the platform widens out, becoming 4 miles wide 



