so On the Structures of Rocks. 



may be undefined, and so may the resulting forms. Thus the 

 concretionary structure may bear a real analogy to crystallization, 

 or it may even be supposed a modification of that process. We 

 know that it exists ; we are ignorant alike of the laws of both. 

 But that they have a real connection is proved by the phenomena 

 above-recited, respecting the smaller spheroidal structures. la 

 these, it is absolutely impossible to define the point at which the 

 one ceases and the other commences. The radiated crystalline 

 spherula passes into one consisting of solid unradiated concentric 

 crusts ; and that again, in a manner equally gradual, into a solid 

 sphere without any internal structure. 



I know not that at present any further light can be thrown on 

 this obscure subject, which I willingly leave to other hands and to 

 further information. As far as relates to the magnitude of some 

 of the masses considered as concretionary, there is no cause for 

 objections. We can even see no reason why nature might not have 

 produced a crystal of mountainous bulk, provided the requisite 

 circumstances were present. The polar tendency of crystalliza- 

 tion is often prolonged through various obstacles, as is daily seen 

 in minerals : it may be protracted indefinitely along the atoms of a 

 compound mass, as is evinced by the granite vein in Coll, which I 

 have elsewhere described. The tendency to form certain concre- 

 tions may equally be unlimited; and thus it need excite no surprise 

 if even the granitic lamince of the Alps, which have been supposed 

 the products of an extensive but disturbed stratification, have been 

 produced by a concretionary arrangement analogous to crystalli- 

 zation. 



