1869.] 291 [Shaler. 



comparable to this, where the concentric divisions being the original 

 planes of stratification, have been thrown into a boss -like form by 

 the action of elevatory forces. But however true it may be that 

 nature forms on a large scale true and unbroken domes, a question 

 which cannot be discussed here, there is probably no need of disprov- 

 ing the dome theory, as applied to the masses of from fifty to five 

 hundred feet in diameter, where this concentric structure is most 

 manifest. 



That concentric structure is induced by the action of forces which 

 operate on the external surface of the masses which exhibit indica- 

 tions of this feature, is sufficiently proven by the fact, that wherever a 

 surface of rock, of the character which enables it to fracture in this 

 way, has been laid bare by recent geological changes, concentric 

 fractures, though previously wanting, soon become distinctly mani- 

 fested. 



The most unquestionable evidence of this fact is to be found when- 

 ever trap dykes have been excavated by the action of atmosphere 

 or waves, thus exposing to external influences some portions of the 

 massive rocks which had been previously sheltered from the air. 

 It is often possible to trace the same dyke, from a point where it re- 

 mains almost unworn, to where it has been eroded to the depth of 

 many feet; and in such cases it may frequently be seen that the fis- 

 sure formed in the massive rock exhibits on its sides distinct evidence 

 of the concentric lamellation, which is most clearly shown where the 

 walls have been the longest exposed to the weather, and becomes less 

 and less plain as Ave ajiproach the not yet eroded trap beneath which 

 it evidently does not exist. When the side walls of the dyke have 

 become worn down from the vertical position, so that they form 

 shelving slopes of a low angle, the concentric structure is still clearly 

 visible, and is often the means by which disintegi-ating agents find 

 their way into the massive rock. 



It is not very easy to see in what way external forces operating on 

 a massive rock could fissure it in this concentric form ; it may be the 

 product of a number of forces acting together, but it is probable that 

 the chief agent is to be found in the changes of temperature to which 

 the sui'face has been exposed. A rock surface on our sea-shores and 

 hill-tops is chilled in winter to thirty degrees below zero, and under 

 the midsummer suns absorbs enough heat to raise the temperature 

 often to one hundred and twenty and one hundred and twenty-five 

 degrees, giving a variation of about one hundred and fifty degrees 

 extending over the six months of the half year. In addition to this 



