142 DYNAMICAL GEOLOGY. 



time, at very long intervals, there occur very great erup- 

 tions. If the volcano be of the quiet type, the whole top 

 of the cone is melted, and, after eruption, is ingulfed ; 

 or if of the explosive type, the whole top of the cone is 

 blown into the air, and the mountain is disemboweled. 

 In either case a yawning chasm many miles in extent is 

 left. 7. Within this great crater, by subsequent erup- 

 tions, is built up a smaller cone, and within this again 

 often still smaller cones.- Thus volcanoes often have 

 about their present eruptive cone a great surrounding 

 rampart. This rampart is the remains of the great 

 crater. In Vesuvius (Fig. 75), Mount Somma, s, is the 



FIG. 75. Section of Vesuvius, vv, Vesuvius cone ; , Mount Somma ; ', other side 

 of Somma overflowed by lava from Vesuvius. 



remains of such a great crater, the other side of it being 

 broken down, and now covered by flows from the present 

 crater. 



Crater Lake. An excellent example of this structure 

 is found in Crater Lake. This beautiful lake, with its 

 splendid blue waters, occupies a yawning chasm on the 

 top of an extinct volcano in southern Oregon. The lake 

 itself is about six miles in diameter and 2,000 feet deep 

 (the deepest lake on the American continent), and is sur- 

 rounded by almost perpendicular walls, 1,000 to 2,200 feet 

 high. From the midst of the blue waters, but nearer one 

 side, there rises a beautiful island (Wizard Isle), 800 

 feet high, which is in fact a cinder cone with a small 

 crater atop. Fig. 76 gives an ideal section of the lake 

 and island, and also, in dotted outline, the supposed form 



