Nearly all these mountains have craters either on their summits or on their sides. They are extinct volcanic mountains, that were, for the most part, thrown up during the Tertiary Geological Period, so that they have all been greatly affected by erosion.
One of the most remarkable of the above volcanic mountains is Mt. Mazana, in Oregon. This mountain has on its summit an approximately circular cavity from five to six miles in diameter, that is occupied by a lake of water known as Crater Lake. This lake is 6,239 feet above the level of the sea, and has a depth of 1,975 feet. It is surrounded by nearly vertical walls ranging from 900 to 2,200 feet deep, so that the vast caldera of which this great depression consists has a depth of at least 4,000 feet.
Mt. Pitt, situated about sixty miles north of Mt. Shasta, in southern Oregon, has a regularly shaped volcanic cone, and the remnant of a crater at its summit. The Three Sisters and Mt. Jefferson lie to the north of Mt. Pitt. Like the others they are ancient volcanic mountains. But little is accurately known concerning them.
Mt. Hood, 11,225 feet high, rises from the crest of the Cascade range in Northwest Oregon, about twenty-five miles south of the Columbia River. Mt. Hood is an exceedingly majestic mountain. At its summit there are only portions of the walls of the original crater. When ascended in 1888, streams of sulphur vapor were escaping from fumaroles on its northeastern slopes, at an elevation of 8,500 feet above the sea.
Mt. Adams and Mt. St. Helen's lie to the north of Mt. Hood. Mt. Adams about sixty miles to the north, and beyond this, Mt. St. Helen's. Accurate information concerning the summit of Mt. Adams is still lacking. Mt. St. Helen's in Washington has more of a conical summit. Russell states that according to frontiersmen, St. Helen's has been in a state of activity within the past fifty years. A French-Canadian asserts that the mountain was in actual eruption during the winter of 1841-43, that at this date the light from the volcano was sufficiently bright to enable one to see and pick up a pin in the grass at midnight near his cabin some twenty miles distant. Mt. St. Helen's was ascended in 1889, when fumaroles were found on the northeast side.
Mt. Rainier in Washington is plainly visible from Puget Sound. It is a most magnificent mountain. The summit has a bowl-shaped crater, of an almost perfectly circular form. The inside of the crater, when last ascended, was filled to within thirty or thirty-five feet of its rim with ice and snow. There was, however, evidences of heat, since numerous jets of steam were seen issuing from its interior rim.
Mt. Baker, Washington, is the northernmost of the volcanoes of the Cascade Mountains, south of the boundary line between the United States and Canada. But little is known of this mountain. The summit appears as a conical peak from Puget Sound, so that its form would seem to show that it is of volcanic origin. According to Gibbs, officers of the Hudson Bay Company, as well as the Indians, declared that Mt. Baker was in eruption in 1843, when it broke out at the same time as Mt. St. Helen's, covering the country with ashes.
There are but few volcanoes in the Rocky Mountains which extend from north to south through the United States at a considerable distance to the east of the Sierra Nevadas and Cascade Ranges. The Spanish Peaks, situated in the southeastern part of Colorado about sixty miles south of Pueblo, are the remains of ancient volcanoes. Two of the most prominent of these peaks rise from 12,720 to 13,620 feet above the sea.
We shall make no effort to attempt to describe the volcanic mountains that may exist in those portions of the Rocky Mountain Ranges or the Cascade Range lying in Canada. Comparatively little is known of them, but inasmuch as volcanic activity has been manifested in Alaska, it would seem highly improbable, as Russell remarks, that volcanoes should suddenly cease at the northern boundaries of the United States and then begin again at the most southern part of Alaska. It will be sufficient to say that Mt. Edgecome, situated on an island in the neighborhood of Sitka, is of volcanic origin, and that the Aleutian Islands, beginning at Alaska on the east at the head of Cook's Inlet, extend westward through the Peninsula of Alaska to the Peninsula of Kamtschatka for a distance of nearly 1,600 miles. This belt, which is called by Russell "the Aleutian Volcanic Belt," contains numerous volcanoes that are known to have been active in historical times.
Mt. Wrangell, on the Copper River, 200 miles northeast of the head of Cook's Inlet, is a lofty volcanic mountain that is said to have been in eruption in 1819, and at the time of last report was still throwing out columns of steam. While much remains to be ascertained about the volcanoes of the Aleutian Islands, it would appear that there are active volcanoes on twenty-five of these islands, on which some forty-eight craters have been found. Eruptions are common in the district.