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Show MOUNT HILGAED. 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 Moraine 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 particular spot as possessing a more distinctly focal character than the others. The outpours appear to have occurred all along the line, with an approximation to uniformity, or possibly with a gradual increase of magnitude and frequency, as we approach the summit of Mount Hilgard. From that headland southward the top of the platform widens out, becoming 4 miles wide |