It can of course very seldom happen that actual mining operations, like those undertaken in the case of the Kammerbühl, will be resorted to in order to determine the structure of volcanic mountains. Geologists have usually to avail themselves of less direct, but by no means less certain, methods than that of making artificial excavations in order to investigate the earth's crust. Fortunately it happens that what we cannot accomplish ourselves, nature does for us. The action which we call 'denudation' serves as a scalpel to dissect volcanic mountains for us, and to expose their inner recesses to our view. Many portions of the earth's surface are complete museums crowded with volcanic 'subjects,' exhibiting every stage of the process of dissection. In some, rains and winds have stripped off the loose covering of cinders and dust, and exposed the harder and more solid parts--the skeleton of the mountain. In others, the work of destruction has proceeded still further, and slowly wearing rivers or the waves of the sea may have cut perfect, vertical sections of the mountain-mass. Sometimes the removal of the materials of the volcanic mountain has gone on to such an extent that its base and ground-plan are fully exposed. It only requires the necessary skill in piecing together our observations on these dissected volcanoes, in order to arrive at just views concerning the 'comparative anatomy' of volcanoes. As the knowledge of the structure of animals remained in the most rudimentary condition until the practice of dissection was commenced, so our knowledge of volcanoes was likewise exceedingly imperfect till geologists availed themselves of the opportunities afforded to them of studying naturally dissected volcanic mountains. In some cases we may find that the sea has encroached on the base of a volcanic hill, till one half of it has been washed away, and the structure of the mass to its very centre is exposed to our view. Thus in fig. 6 (page 43), it will be seen that there lies in front of Vulcano a peninsula called Vulcanello, consisting of three volcanic cones, united at their base, with the lava-streams which have flowed from them. One half of the cone on the left-hand side of the picture has been completely washed away by the sea, and a perfect section of the internal structure of the cone is exposed. The appearances presented in this section are shown in the sketch, fig. 35. Some portions of the face of this section are concealed by the heaps of fragments which have fallen from it, but enough is visible to convince us that three kinds of structures go to make up the cone. In the first place, we have the loose scoriæ and lapilli, which in falling through the air have arranged themselves in tolerably regular layers upon the sides of the cone. [Illustration: Fig. 35.--Natural section or a Volcanic Cone in the Island of Vulcano.