Some of the questions here asked will be further dealt with in later pages in connection with such geological evidence as can be produced regarding them. But it will be found that at every step in the endeavour to ascertain the origin of volcanic phenomena difficulties present themselves which are now and may long remain insoluble. I. Vents of Eruption It is a general belief that the first stage in the formation of a volcano of the Vesuvian type by the efforts of subterranean energy is the rending of the terrestrial crust in a line of fissure. Some of the most remarkable groups of active volcanoes on the face of the globe are certainly placed in rows, as if they had risen along some such great rents. The actual fissure, however, is not there seen, and its existence is only a matter of probable inference. Undoubtedly the effect of successive eruptions must be to conceal the fissure, even if it ever revealed itself at the surface. What is supposed to have marked the initial step in the formation of a great volcano is occasionally repeated in the subsequent history of the mountain. During the convulsive shocks that precede and accompany an eruption, the sides of the cone, and even sometimes part of the ground beyond, are rent open, occasionally for a distance of several miles, and on the fissures thus formed minor volcanoes are built up. It is in Iceland, as already stated, that the phenomena of fissures are best displayed. There the great deserts of lava are from time to time dislocated by new lines of rent, which ascend up to the surface and stretch for horizontal distances of many miles. From these long narrow chasms lava flows out to either side; while cones of slag and scoriæ usually form upon them. This interesting eruptive phase will be more fully described in the chapters dealing with the Tertiary volcanic rocks of Britain.