246 THEORY of the EARTH. 



THE fadl to be proved with regard to thefe iron-ftones is this, 

 That they have acquired their folid (late from fufion, and not 

 in concreting from any aqueous folution. 



To abridge this difquifition, no argument is to be taken 

 from contingent circumftances, (which, however, are often 

 found here as well as in the cafe of marbles) ; fuch only are to 

 be employed as are general to the fubjecT:, and arife necefTarily 

 from the nature of the operation. 



IT will be proper to defcribe a fpecies of thefe ftones, which 

 is remarkably regular in its form. It is that found at Aberlady 

 in Eaft Lothian. 



THE form of thefe iron-ftones is that of an oblate or much 

 comprefled fphere, . and the fize from two or three inches dia- 

 meter to more than a foot. In the circular or horizontal fec- 

 tion, they prefent the moft elegant feptarium * ; and, from 

 the examination of this particular ftructure, the following con- 

 clufions may be drawn. 



Fir/t, THAT the fepta have been formed by the uniform 

 contraction of the internal parts of the ftone, the volume of 

 the central parts diminifhing more than that of the circum- 

 ference ; by this means, the feparations of the ftone diminifh, 

 in a progreflion from the centre towards the circumference. 



id, THAT there are only two ways in which the fepta muft 

 have received the fpar with which they are filled, more or lefs, 

 either, jirft, By infinuation into the cavity of the fepta after 

 thefe were formed ; or, zdly, By feparation from the fubftance 

 of the ftone, at the fame time that the fepta were forming. 



WERE the firft fuppofition true, appearances would be ob- 

 fervable, fhewing that the fparry fubftance liad been admitted, 

 either through the porous ftruaure of the ftone, or through 

 proper apertures communicating from without. Now, if either 

 one or other of thefe had been the cafe, and that the ftone had 

 .been confolidated from no other caufe than concretion from a 



difTolved 



* Plate I. 



