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266 ON THE MINERALOGY 



ments of wacke, contained in a paste of the same substance in 

 a state of decomposition ; it is of a very fine grain, very soft, 

 and almost friable. The other is composed of fragments of 

 wacke, but more compact, and of globular pieces of basalt. 

 When these globules are broken, the interior is occupied by 

 geodes of crystallised apophyllite, accompanied with capillary 

 mesotype, sometimes decomposed and reduced to powder, in 

 ^^ ^ which state it is known by the name of Earthy Zeolite. These 



^' ' * *N ^^'^ *^^ only minerals I found in this globular basalt!' The apo- 

 Xt^phyllite 1 never observed in the other variety of trap-tuff, in 

 j.^" Jl-^ which I discovered no simple mineral whatever, except some . 

 i? , very small geodes of radiated zeolite. I shall distinguish the 



^ '-^v one by the name of Trap-tuff, and the other by that of Basalt- 

 tuff. The last appears to me to be the oldest of the two, and ^. JM 

 , occurs, wherever I saw it, under the other. If the tuff be en- 

 ^ " tirely absent, then the Amorphous Basalt occupies its place, 



and on it rests the Amygdaloid the paste of which is of a red- 

 dish-brown colour. It is the amygdaloid of this colour in 

 ^ which the greatest number of minerals occur, such as stilbite, 



mesotype, quartz, calcedony, and igloit. When exposed to 

 ^ ^^ ^^ '' *; the action of the weather, this rock becomes extremely fragile, 

 ^'*'* „ and falls in conchoidal fragments, almost like bole. It occa- 



^ 'It^^ sions, particularly in spring season, by reason of its feeble co- 

 ^ ^ hesion, immense devastation. Rent by the effects of tiie se- i 



. ^ lA vere frosts of vvinter, it falls in huge blocks into the valleys, . a'J^ * 

 •^M, ' when the basalt, deprived of its support, is precipitated in' "sj^ 



'' y . enormous masses, and to such an extent, that rivers are often v' 



V ^ M, impeded in their course, and the whole neighbourhood laid *v WW 



*| '" under water. Over this amygdaloid, a mass of ferruginous j 



'^^ ^ ^'^j, , clay occurs, similar to the Eisenthon of the Germans, which -^ * v 

 ^^" i' '"approaches to the jaspery oxide of iron. This is again covered '"?•' 3 

 ■ \ ^ A*^ ^by amorphoug basalt, separated from columnar basalt, which-, 



usually \' 



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