Irish Volcanoes. 187 



most ancient of the historic records of old Ireland. Certainly 

 also if we could know all that the oldest Phenicians, or yet 

 earlier visitors, have seen in those times when the native tribes 

 inhabited the crannoges, and dwelt in lake-villages, killing the 

 wolf, and even the great-horned elk, with flint-tipped arrows 

 and stone hatchets, we should be left in equal ignorance con- 

 cerning these phenomena. But it is none the less certain that 

 the events took place, and that, at a period historically remote 

 enough, but geologically modern, the whole north-east of 

 Ireland and the western islands of Scotland were parts of the 

 bottom of an ocean, beneath which subterranean thunders 

 growled, and occasionally and at no long intervals the up- 

 heaving force rent the solid rock, and poured through the 

 fractured crust a fluid glass precisely identical with the lava 

 floods of Vesuvius and Hecla. But as the molten mass was 

 always thrust up under the pressure of a considerable depth of 

 water, it was spread evenly over the ocean floor in a compara- 

 tively thin sheet, and both water above and chalky mud below 

 being exceedingly bad conductors of heat, the thicker deposits 

 must have taken a very long time to cool. Now it is well 

 known from observation that the sand at the bottom of an 

 iron or other furnace left exposed to intense heat for a long 

 time, solidifies into a kind of glass, and when taken out breaks 

 up into small groups of columns. These are the simple and 

 inevitable results of the change produced by long- continued 

 heat, and slow cooling. The columns are tolerably regular, 

 and always tend to the six-sided form. A similar though less 

 perfect construction is seen wherever melted rock cools slowly, 

 and the more gradual the passage into a solid state — the more 

 uniform the conditions, and the heavier the pressure under 

 which the operation is carried on — the more compact is the 

 resulting mass, and the more perfect the concentric and 

 columnar structure. For after all the latter is the result of 

 the former. When a simple mineral is allowed to arrange 

 itself naturally, it crystallizes, or forms into a group of distinct 

 angular shapes, often beautiful, and always reducible to the 

 same mathematical form. When a mineral mass or rock, 

 made up of several minerals not very regularly put together, is 

 melted and cooled again, it tends to form into rounded lumps 

 or spheres, and if allowed to form very slowly, these seem 

 made up of successive coats, like the coats of an onion. 

 After long exposure to the weather, this tendency always 

 shows itself in such rocks as lava. In cases where the 

 conditions are very uniform, these spheres seem to have 

 been checked in their tendency to form perfect spheres by 

 pressing against each other. Since, however, in a layer of 

 shot or round solid balls, each one touches and is touched by 



