Crystalline Structures of Rocks. 75 



the result. If, on the contrary, sufficient time be granted, the 

 consequence is the generation of numerous minerals, producing 

 not only the granitic structure, but the porphyritic also. It is not 

 necessary here to argue the question of graphic granite, which 

 was originally brought forward to prove the same conclusion; since 

 the basis of the reasoning is the same. 



The last structure to be noticed is the amygdaloidal, and it is 

 preferable that it should be examined here, that the whole of the 

 subject of structure, as far as it forms an object of geological 

 theory, should be seen in one condensed view. That its nature 

 has been a subject of dispute, is an additional reason for intro- 

 ducing it among other subjects equally matters of controversy. If 

 the view of its cause here to be given shall be admitted, it will be 

 seen, however, that it has no proper title to rank among the modi- 

 fications of the concretionary structure. 



The variety of minerals contained in the cavities of amygdaloids, 

 does not form any part of this enquiry ; but it is necessary to state, 

 that this structure is limited to the trap family and to the volcanic 

 rocks. It is universally admitted, that the cells of volcanic scoria 

 have been produced by aeriform matters disengaged during the 

 process of fusion. Similar cells are found in the trap rocks, as I 

 have elsewhere shown (Western Islands), and these rocks have also 

 been produced in the same manner. 



Now the cells which, in either of these classes of rock, contain 

 the amygdaloidal minerals, differ in no respect in form and dispo- 

 sition from those that are empty ; and if their internal surfaces be 

 examined, it will be found that they are often coated with a similar 

 vitreous varnish. These cavities are not always filled with the 

 minerals which they contain ; but present vacuities, in which it 

 frequently happens that the crystalline terminations of the mine- 

 rals are defined. In the next place, two minerals, or even more, 

 are sometimes found in one cavity ; in some cases interfering with 

 each other's forms. Lastly, similar cavities occur in the same rocks, 

 sometimes of considerable size, yet connected by a gradation of 

 magnitude with the smaller cells. These seem to be the circum- 

 stances most essential to the ars-ument under review. 



