180 ON THE CONCRETIONARY AND 



It is now important to remark, that these last- 

 named spherules, wherever the forms are most perfect, 

 present a concretionary structure passing into one 

 which is decidedly crystalline. Brilliant fibres ra- 

 diate from the centre, and are repeated at intervals 

 so as to form successive concentric crusts of the same 

 nature; or else these crystalline spheres are surround- 

 ed with crusts in which oo fibrous structure can be 

 traced. There is thus a transition from the most 

 perfect crystalline to the most imperfect concretionary 

 spherule. 



In attempting to explain these appearances, it is 

 interesting and important to observe how these sphe- 

 roidal crystallized forms coincide with those which 

 occur in melted glass under certain circumstances, 

 and how accurately they resemble the analogous ap- 

 pearances produced in Mr. Watt's well known ex- 

 periments, where the arrangement was produced after 

 the fused trap had lost its fluidity. Thus it is equally 

 easy to comprehend how a solid mass of any of the 

 above named rocks, softened, if it is necessary to sup- 

 pose so, without fusion, or otherwise under the long 

 continued influence of heat, might have assumed a 

 similar species of structure. As also, in one of these 

 cases, there is a gradual progress from the most per- 

 fect crystalline to the most imperfect concretionary 

 arrangement, there can be no reason to doubt, that 

 in every case, the latter may also be produced by the 

 same causes. It should here lastly be added, in con- 

 firmation of this theory, that a spheroidal structure 

 of a similar nature exists in the Trap of the Shiant 

 isles. Those of the granite, the porphyry, and the 

 greenstone, as it is called, of Corsica, will naturally 

 occur to every geologist. 



A spheroidal structure, terminating also, by wasting, 



