CADEK IDIS. BASALT IN AUVERGNE. 



143 



tenacity of the lava, prevented its escape, it would be compressed, 

 and form cavities, or air bubbles, at the bottom of the melted mass. 

 In other instances, where the fluidity of the lava permitted the steam 

 from below to escape through it, the mass would be compact and 

 form solid basalt, or greenstone. It might sometimes happen that 

 water would be enclosed in the cavities of the mass, which is found 

 to be the case in some basalt rocks. 



Thus, according to the different circumstances of pressure from 

 the depth of the ocean, and from the tenacity of the melted mass, 

 Sir G. Mackenzie, supposes that porous and vesicular lava, or com- 

 pact basalt, might be formed from the same eruption ; or the mass 

 might be porous below and compact above. 



As Iceland is at present the seat of active volcanoes, and as sub- 

 marine volcanoes are forming rocks near the shores of that island, 

 Sir George Mackenzie's explanation of the causes which have pro- 

 duced the various appearances in the basaltic ranges of that island, 

 seems highly probable. In Sicily, the connection of basaltic with 

 volcanic rocks has been clearly established by Ferrara, Professor of 

 Natural Philosophy at Catania. 



In the vicinity of Clermont Ferrand in Auvergne, a thick bed of 

 basalt has once covered an extensive tract of country; it rests upon 

 a bed of volcanic tufa, and the latter frequently covers beds of fresh 

 water limestone. This bed of basalt, and the subjacent tufa and 

 limestone, have evidently been furrowed and excavated by the same 

 causes, which have excavated valleys in other parts of the world ; 

 hence the basalt occurs, forming isolated caps on many of the moun- 

 tains. In some parts a gradation may be traced in the same bed 

 from a compact basalt, similar to that of Arthur's Seat near Edin- 

 burgh, to porous basalt, approaching more or less to the state of sco- 

 riaceous lava. But the basalt of Auvergne belongs evidently to vol- 

 canic products, and will be described in the chapter on volcanoes. 

 It may be proper to remark, that as the basalt of Auvergne covers 

 beds of fresh water limestone, which belongs to the tertiary strata, 

 its age is evidently posterior to that formation of limestone, which is 

 regarded as the most recent. 



Basalt sometimes presents a globular structure, globes of hard ba- 

 salt being imbedded in a mass of basalt of a softer kind. 



Wacke or earthy basalt has frequently a greenish or reddish brown 

 colour ; it often contains cavities which are generally filled with nod- 

 ules of agate, or with zeolite or calcareous spar. The agates are 

 composed of concentric layers, and have apparently been formed by 

 siliceous infiltration, depositing successive coats within each other, 

 until the cavity is filled up. Basaltic rocks of this kind are called 

 amygdaloids. The Hill of Kinnoul, in the vicinity of Perth, is form- 

 ed of basaltic amygdaloid, containing agate nodules in great abun- 

 dance, of various dimensions and beautifully striped. At Woodford 

 Bridge, in Gloucestershire, there is a low rock of amygdaloidal 



