188 Irish Volcanoes. 



the adjacent solid "balls at six points, leaving interspaces (a 

 fact very easily proved by experiment), it is clear that if the 

 balls were soft, and lateral pressure uniform, they would be 

 squeezed into six-sided figures with rounded terminations. 

 But if there should be a succession of such layers, the pressure 

 at top and bottom being considerable, and the balls plastic, a 

 series of vertical six-sided columns would result — these columns 

 being divided, and the divisions flattened, but still perhaps 

 showing more or less of the concentric structure. This is pre- 

 cisely the case at the Giant's Causeway, and in numerous other 

 instances where lava has been erupted and cooled under similar 

 conditions. This explains the saucer- like top of some of the 

 columns, and the ball-like fitting of others into the hollow 

 socket adjoining. Thus also we account for the various chairs, 

 with their fanciful names, already alluded to, and for the organ, 

 the chimney, and other instances of longer and more regular 

 columns ; and thus again we are able to understand the repeti- 

 tion of the columnar structure in various parts of the same bed 

 of rock. 



It has been found that from various parts of the cliff of 

 basalt near the Causeway, curious specimens of minerals may 

 be obtained. Among these are beautiful chalcedonies and 

 opals, rich agates, and not a few less durable, but even more 

 beautiful crystals. These are usually found in the bubbles or 

 cavities of particular bands of rock, not columnar. Some of these 

 bands are crowded with innumerable cavities, generally small ; 

 others contain larger specimens, but not so many in a given 

 space. The contents of the cavities are the gems of the 

 Causeway, and are identical with the gems of Lough Neagh. 

 Some of the best specimens are found close to a bed of 

 altered woody matter once worked for fuel. Others are from 

 a bed near a deposit of fine red ochre. It is not difficult to 

 explain the origin of these minerals. Where the lava has 

 been poured out under deep water, the conditions of cooling 

 are regular, and the rate slow. In these cases there are 

 either no cavities or very few, but the rock is broken up into 

 columns, and then again into slices. Where, however, the 

 water has been shallow, and the cooling much more rapid, as 

 must have been the case when the rock rests on altered trunks 

 of trees, the lava has become more like pumice, and full of 

 bubbles. Originally, perhaps, these were full of gas, but they 

 have since been filled with crystals derived from the surrounding 

 rock. These numerous blebs and cavities completely alter the 

 appearance and texture, and even affect the colour of the rock, 

 which there much more resembles the lava of Vesuvius than 

 the compact mass of the Causeway columns. 



The reader will now understand the theory of the Giant's 



