production of the Prismatic Structure of Basalt. 219 



rigidity was sufficient to prevent the subsequent distortion in 

 the forms of the prisms without preventing those minute mole- 

 cular movements by which slaty cleavage is developed in rigid 

 bodies under severe pressure. 



We have thus traced the production of prismatic structure, 

 and all the leading phenomena which it presents in nature, to a 

 single and simple cause, namely contraction by cooling, the ad- 

 mitted physical laws governing which sufficiently account for 

 the hexagonal as the normal form of the prisms, for the diame- 

 ters assumed by the prisms, for the variably cupped form and 

 the direction taken by the convexity of the transverse joints 

 and the variability of their distances apart, for the various direc- 

 tions in which the axes of straight prisms are found in nature, 

 and for the production of prisms whose axes are variously cur- 

 vilinear. 



Upon the principle, then, that that solution of any natural 

 problem which accounts, upon admitted principles, for all the 

 phenomena, and introduces as consequences no new facts not 

 accountable for, must be the true one, the writer ventures to 

 believe he has solved this problem which for nearly ninety 

 years has perplexed physical geology. 



If the principles here enunciated be admitted, it will be readily 

 seen that they present a powerful and valuable guide to the field- 

 geologist for deciphering the hitherto perplexing phenomena 

 presented by basalt, as well as many of those produced by it 

 upon adjacent rocks. 



It would seem therefore unnecessary to enter upon any for- 

 mal refutation of those very crude and ill-thought-out notions 

 which attribute prismatic structure to the squeezing together of 

 globular or spheroidal masses like the "onion-stone" of Antrim, 

 or like the superimposed spheroids which are to be found in 

 all large basaltic regions. Such spheroids are found not con- 

 fined to basalt, but existing as the residues of chemical decom- 

 position and weathering in all rocky formations, presenting a 

 more or less distinct horizontal stratification with vertical planes 

 of separation. Such instances are familiar, not only in granitic, 

 gneissose, and porphyritic rocks, but in many of a far softer 

 character, argillaceous or calcareous. The superposed columns 

 of orbicular masses of onion-stone prove themselves by every 

 indication to be the decomposing remains of what were once 

 jointed prismatic columns of basalt. 



The exterior of each spheroid is friable or incoherent, cohe- 

 rence increasing as we advance toward the centre of the sphe- 

 roid, which presents no nucleus of foreign matter or any in- 

 dication that the spheroid has been produced by what have been 

 vaguely called concretionary forces. The little irregular pris- 



