BASALTIC ROCKS. 



. 90. The fluid basalt often assumes the form of prismatic columns 

 in the process of crystallisation, consequent upon slow cooling. 

 Mr. Gregory Watt imitated this artificially by reducing 7 cwt. of 

 Dudley basalt to fusion and causing it to cool slowly, when 

 globular masses were formed, which gradually enlarged and 

 pressed one against the other, until they forced themselves into 

 regular columns, resembling in all respects those of natural 

 basalt. 



, In some places basalt forms vast plateaux of considerable thick- 

 ness, in others it is found in detached sheets of less extent, at 

 points of mountains, more or less distant one from the other, and 

 at the same level, as if it had originally been a single sheet and 

 had been disrupted, by the convulsions of which the mountains 

 have been the result. 



In some cases the basalts form isolated masses or mounds rising 

 in the midst of plains, altogether removed from all similar forma- 

 tions. They are also often found in veins in the strata of the 

 earth, like those of minerals. They sometimes also present them- 

 selves as extensive walls, or in a series of separate mounds having 

 a common direction. When basaltic rocks are presented in the 

 form of sheets or mounds, the upper part is generally composed of 

 porous cellular scoriform matter, irregularly divided, and termi- 

 nated below by a plane surface, sensibly horizontal. When the 

 mass is composed of several layers, these layers are separated one 

 from another generally by thin beds of rapilli.* 



91. Basaltic deposits are much more extensively scattered over 

 the surface of the globe than those of ordinary volcanic origin. 

 Unlike volcanic products, they are not confined to particular cen- 

 tres of action, but appear to have been produced wherever the 

 terrestrial crust, yielding to the pressure from below, was rent 

 so as to give issue to the fused matter. In the British Isles ba- 

 saltic products are found in various places, and more particularly 

 in the north of Ireland and Scotland. In France they are found 

 everywhere from the northern part of Auvergne to Montpellier, 

 and even to Toulon. On the borders of the Rhine they extend 

 from the Ardennes to Cassel, and are continued eastward into 

 Saxony, Bohemia, and the surrounding countries. They prevail 

 to a great extent in Iceland, are recognised in the West India 

 Islands and St. Helena, in the island of Ascension, and in almost 

 all the islands of the southern ocean. 



92. The tendency of these rocks to form themselves into pris- 

 matic columns has more especially excited the attention of the 

 curious. In some cases all the prisms converge to the summit of 



* Volcanic dust. 



77 



