JOINTS IN VOLCANIC ROCKS. 43 



Dore in the Auvergne, the pitchstone of Arran, the felstone of Cader 

 Idris, and phonolite of the Rochte Sanadoire in the Auvergne. Mr. 

 Koch has stated, that when some slags are cooled under water they 

 also assume a columnar structure. The hexagonal structure of ice, 

 haematite, and quartz would seem to be connected with the fact, 

 that those minerals crystallise in the hexagonal system, and circum- 

 stances have favoured their division into hexagonal prisms. 



But the prevalent columnar structure of basalt is of an altogether 

 different nature. The surface or the floor of the lava stream cooled 

 uniformly, and therefore contracted, so that the cracks appeared near 

 the surface or base, and penetrated deeper and deeper as the cooling 

 progressed ; sometimes leaving an undivided portion in the middle of 

 a thick lava-flow. And it is extraordinary that these cracks always 

 form an angle of about 120 with each other, so that the entire mass 

 of rock is split up into six-sided columns. Both the augite and labra- 

 dorite which form basalt, belong to the monoclinic system of crystal- 

 lisation ; and we think it possible that the angles of the skeleton-crystals 

 of the augite and felspar have determined the angle of the division 

 planes splitting basalt, causing the columns to take a six-sided figure, 

 rather than any other form, so that the structure itself is essentially 

 crystalline. 



When basalt has been exposed to the weather, as in the Giant's 

 Causeway and in Staffa, the columns are often found to be divided 

 transversely by joints, which have been compared to the joints in the 

 back-bone of a shark. And sometimes the outer layer of each of these 

 short pieces scales off, showing an internal concentric structure. This 

 is beautifully seen at the grotto called the Kaskeller, near Bertrich, 

 by the Moselle, where the columns look as though built of Gouda 

 cheese. But a similar though irregular joint-structure may be seen 

 almost as well, on a small scale, in the country above Tobermory in 

 Mull, where many thin concentric layers of compact basalt will scale 

 off from the irregular blocks, leaving in the centre a small, rough, 

 more crystalline ball, with the texture of dolerite. Up the Ehine 

 from Bonn to Andernach, basalt columns do not usually show trans- 

 verse divisions, but are often enormously long, and have to be broken 

 into convenient lengths for the various building purposes for which 

 they are quarried. 



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UNIVERSITY 



