76 Dr. Mac Culloch on the Granite Tors of Cornwall . 



own island of Arran (that little abridgement of the world) nodules of 

 spherical granite are found in the valleys which descend from Gcat- 

 field, decomposing on the surface in crusts, and marking decidedly 

 the very construction which my supposition requires, in a much 

 greater degree than is requisite for the purpose. Similar granite 

 balls have been seen in other places, so that their existence is well 

 ascertained, and this is one of numerous instances, where the decom- 

 position of a rock gives us most useful information with regard to 

 its structure, and where without that aid, we should never have 

 divined the secret of its formation. It is certain that these balls, now 

 rendered spherical by decomposition, have been quadrangular masses, 

 and hence we may step, without any great hazard of unsound 

 footing, to this general conclusion, that these masses of granite, 

 Vv'hich show marks of wearing on their surface with rapidities pro- 

 portioned to their distance from a central point, have had their 

 hardness, and probably their crystallization or formation, determined 

 from that centre. 



The analogy of this circumstance to the similar balls formed in 

 basaltic rocks is illustrative of both the cases, and probably both will 

 equally tend to confirm the opinions which have been held relating to 

 the igneous origin of these substances. Thus, if for the sake of 

 argument I may be allowed to assume that granite is of igneous 

 origin, it will be easy to explain the peculiar appearances exhibited 

 by that formation of granite, which, like those of Cornwall and Arran 

 and many others, is separated into cuboidal masses. 



Here we must conceive, that in a homogeneous mass of fluid matter, 

 crystalUzation had commenced from numerous centres at the same 

 time. "While there was yet space for the formation of successive 

 solid deposits round any set of these imaginary centres, a sphe- 

 rical or spheroidal figure would be the result. As the surfaces 



