THE CRUST OF THE EARTH. 



a mound, which thus assumes a sheaf-like structure. In others 

 they take the form of close columns with the most picturesque 

 aspect. In others again, these columns, cut off at a certain level, form 

 a sort of mosaic pavement, to which the name of causeway has 

 been given. Of this one of the most magnificent examples is pre- 

 sented in the case of ttye Giant's Causeway, in the north of 

 Ireland. 



93. Examples of similar formations are presented in different 

 parts of Europe, and especially in the Vivarais, in the department 

 of Arddche, in France. A remarkable series of basaltic causeways is 

 there presented on the banks of the river Volant, between Yals and 

 Entraignes, a view of which is given in fig. 31, p. 33. The colon- 

 nades of Chenavari, near R,ochemaure, fig. 34, and the dykes which 

 are near the same place, fig. 35, present examples of other varieties 

 of basaltic forms. 



94. Basaltic rocks, having all the prismatic characteristics above 

 described, are frequently presented in the form of mineral veins. 

 Examples of this are found in the central parts of France, and also 

 on the borders of the Rhine. Most commonly the matter composing 

 the vein is compact or divided by irregular cleavage, but it also 

 sometimes exhibits the prismatic form, the axis of the prisms being 

 horizontal, fig. 36. 



95. When basalts take the form of a mound, the lower part 

 of the mass often presents a multitude of appendages which 



Fig. 34. Colonnade of Chenavari, Fig. 35. Dykes of Chenavari. 



near Rochemaure. 



penetrate like roots into the subjacent earth ; showing that the 

 matter in a liquid state had flowed into the crevices, and 

 moulded itself there. The earth thus in contact with the 

 basaltic mass is often found calcined to a considerable depth, 

 and the vegetable remains which it includes are carbonised. 

 Examples of this are presented upon the cliffs of the plateau of 

 78 



