ygo PETRIFIED SHELLS, FOUND IN THE 



been called stratified, from the parallelism of its planes ; the composi- 

 tion being identical, and, without doubt, cotemporaneous. These changes 

 and passages, from one rock into the other, are so frequent and various, 

 as to render it impossible to refer the most of them to either of the rocks 

 I have abovementioned, as types. I shall, therefore, proceed to describe 

 those which are distinctly marked, and their accompanying minerals. 

 In external appearance, the columnar and semi-columnar basalt closely 

 resembles that of the Giant's Causeway, possessing the same fracture, 

 internal dark colour, and external brown crust. It is equally compact and 

 sonorous. It, however contains, more frequently, crystals of olivine, of 

 basaltic hornblende, and of carbonate of lime. The fusibility of each is 

 the same. Perhaps the basalt of Gawilgerh range, more nearly resem- 

 bles, in every respect, that of the Pouce mountain in the Mauritius. This 

 is, however, of very little importance, since every body who has travelled 

 much in trap countries, knows well what great changes in composition and 

 structure occur even in continuous masses. Among the minerals, calce- 

 dony, and the different species of zeolite, are rarely found in the columnar 

 basalt, but they are of frequent occurrence in that which is semi-columnar. 

 The wacken, or indurated clay« i^ various in character and composi- 

 tion, as the basalt, and, unfortunately, I have no type with which to 

 compare it, as in the case of the basalt of the Giant's Causeway. Its 

 colour varies with its constituents, but is most usually gray. It is easily 

 frangible, very frequently friable, and is almost always porous and 

 amygdaloidal. It appears to be composed of earthy felspar and horn- 

 blende, with a considerable proportion of oxide of iron. It is always 

 easily fusible into a black scoria, or glass, according to the quantity of 

 zeolite which it contains : of all the trap-rocks, it abounds the most in 

 simple minerals : They are — Quartz. 



Calcedony and calcedonic agates, enclosing crystals of 

 carbonate of lime. 



Common 



