20 LAKES OF NORTH AMERICA. 



In several volcanic regions there are deep, circular depressions, known 

 as " calderas " or " crater-rings,"' which are believed to have been formed 

 by the blowing away of the mountains that once existed above them. A 

 somewhat complete series can be established between craters tliat have 

 been partially broken down by subsequent eruptions, and crater-rings, 

 about which there are in some instances no vestiges of the original craters 

 remaining. There is evidence also in the character of the rocks surround- 

 ing crater rings, and in the adjacent topography, Avhich sustains the 

 hypothesis of their violent origin. 



Two of the largest calderas yet discovered, occur in Italy, and are 

 occupied by Lago di Bracciano and Lago di Bolsena. As described 

 by J. W. Judd, the ' first-named is nearly circular, with a diameter of 

 six-and-a-half miles ; the second, somewhat less regular, has a length 

 from north to south of ten-and-a-quarter miles, and a breadth of 

 nine miles. The only examples of crater-rings in North America 

 that can be referred to are Gustavila lake, Mexico, of which the 

 writer has been unable to obtain detailed information, and Crater lake, 

 Oregon. 



Crater lake has been described by C. E. Dutton,^ and is considered by 

 him as worthy of a high rank among the wonders of the world. It is 

 situated in the Cascade mountains, in northwestern Oregon, thirty miles 

 north of Klamath lake, at an elevation of 6239 feet above the sea. It is 

 nearly circular, without bays or promontories, as indicated on the accom- 

 panying map, Plate 5, and is from five to six miles in diameter. The 

 cliffs of dark basaltic rock encircling it, rise precipitously to heights vary- 

 ing from 900 to 2200 feet, and nowhere offer an easy means of access to 

 the basin within. They plunge at once into deep water, without leaving 

 even a platform at the water's edge wide enough for one to walk on. 

 There are no streams tributary to the lake, and no visible outlet. The 

 waters probably escaj^e by percolation, as the precipitation of the region 

 is in excess of evaporation, and if an escape were not furnished the basin 

 would be filled to overflowing. 



Near the southwest margin of the lake, about half-a-mile from shore, 

 a cinder cone, named Wizard island, rises from the water to a height of 

 645 feet. This cone is regular in form and has a depression in its sum- 

 mit, thus showing at a glance that it is of volcanic origin, and is in fact 

 a miniature crater of eruption. From the base of Wizard island two 



1 Science, vol. 7, 1886, pp. 179-182. Also, 8tli Ann. Rep., U. S. Geol. Surv., 1886-87, 

 pp. 157-158. 



