Ch. VI.] PRIMARY ROCKS. 97 



systems of parallel joints, as already stated, but is also capable 

 of being cleft in directions corresponding therewith ; so that, 

 whether the spheroid be on the large or small scale, it can be 

 subdivided into cuboidal forms. There can be no doubt, 

 however, that these lesser cubes would, on exposure to the 

 air, be also reduced to spheroids; so that it is a difficult 

 matter to decide whether the latter is the original form, or 

 whether it is accidental, resulting from the solid angles 

 exposing a greater surface to the elements ; or, whether the 

 cohesion being weaker in those parts, may oppose less re- 

 sistance to a chemical change. If the decomposition con- 

 tinue after the spheroid is formed, it then takes place 

 uniformly over the whole surface, which consequently ex- 

 foliates in concentric layers. 



Dr. Macculloch has made some interesting observations on 

 this subject, suggested to him by observing in the British 

 Museum, granitic columns from Leptis, in Africa. " I was 

 surprised to find," he says, " that the shafts of these columns 

 were in the act of desquamation, casting off crusts precisely 

 similar to those which occur, in many cases, in natural blocks 

 of granite; but in this instance the detached crust is not 

 decomposed, and appears scarcely changed from its original 

 state, except in tenderness and fragility." * This shows that 

 the atmospheric influence may, under some circumstances, 

 only produce a general and superficial change, just as a metal 

 plunged into a corrosive acid, and immediately removed, will 

 be uniformly corroded ; but if allowed to remain in for some 

 time, the prolonged action of the agent will show that the 

 oxidation has been more energetic at certain points, whereby 

 the crystalline structure of the metallic mass is developed. 



From these facts it is not improbable, that the spheroidal 

 forms, so commonly exhibited by the crystalline rocks, are 

 varieties of original structure, the result of some peculiar 

 mode in which their integrant particles have been aggregated 



* Journal of Science and Arts, vol. xiii. p. 238. 

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