78 Mr. Mallet on a hitherto unobserved Structure 



sizes, imbedded in a matrix of a material similar to their own ; — the texture, 

 cohesion, colour, and frangibility, &c. of the nodule and matrix being so precisely 

 alike, that when two or more nodules are found cohering, on being struck with a 

 hammer they will as readily break through as separate. So entirely identical is 

 the structure of the nodules and matrix, that the existence of the former could 

 never have been ascertained or suspected without the aid of blasting ; bringing to 

 mind the ancient Grecian fable of the statue hidden in the yet unhewn block, 

 revealed by the sculptor's chisel. 



This nodular structure is confined to the more uniform portions of the dyke; 

 or, if it ever existed in the more complex and variegated parts, it appears to have 

 been obliterated by more perfect fusion, — many of the veined and compound 

 specimens presenting the appearance of having flowed in a state of perfect 

 fluidity. 



The nodules are from eighteen inches in diameter to the size of a hazel nut ; 

 and the nodule and its seat, or nidus, may frequently be found and fitted together. 

 They are usually pretty close together, and sometimes appear to have been 

 pressed into actual contact, — their sides being flattened one against the other, and 

 their surfaces of separation irregularly multilateral, like those of coherent bubbles 

 of a mucilaginous fluid. 



Many of the nodules present distinct indications of magnetic polarity. When 

 an imbedded crystal (as of pyrites) occurs in a nodule, at its surface, it is always 

 limited by the latter, or moulded to, and rendered imperfect by the matrix, and 

 vice versa; no instance occurring of a crystal running from one into the other. 



Foreign matter is in general much rarer in the nodule than in the matrix, 

 and rarer in those parts of the dyke which possess the nodular structure, than in 

 those which do not, — ^namely, the veined and variegated portions. In some cases 

 the nodular structure passes insensibly into the solid homogeneous rock. 



This nodular formation is essentially different from any other as yet de- 

 scribed. The nodular, or orbicular granite of Corsica, and the south of France, 

 consists of alternating layers of different crystallized minerals, — the crystals all 

 converging to the centre of each spheroid, and imbedded in a matrix different in 

 substance and arrangement from the nodule : it would appear to owe its formation 

 to chemical rather than mechanical forces, and its origin is probably analogous to 

 that of the quartz nodules found at Bonmahon, County Waterford, or to the 

 formation of agates, hollow spar, onyx, and other minerals of accretion. 



